The invention claimed is:1. A light-emitting element, comprising:a first electrode serving as an anode;a second electrode serving as a cathode;a light-emitting layer provided between the first electrode and the second electrode;a first oxide layer provided between the light-emitting layer and the first electrode; anda second oxide layer provided between the first oxide layer and the light-emitting layer, and having contact with the first oxide layer, whereina density of oxygen atoms in the second oxide layer is lower than a density of oxygen atoms in the first oxide layer,the first oxide layer is made of a semiconductor,the first electrode, the second electrode, the light-emitting layer, the first oxide layer, and the second oxide layer are provided above a face of a substrate, andeither the first oxide layer or the second oxide layer whichever is positioned below the other one, is shaped as a plurality of islands.2. The light-emitting element according to claim 1, whereina portion of at least an upper face of either the first oxide layer or the second oxide layer, whichever is positioned below the other one, is polycrystalline.3. The light-emitting element according to claim 1, whereinbetween the first oxide layer and the second oxide layer, whichever is positioned above the other one, is made of an amorphous oxide.4. The light-emitting element according to claim 1, whereinthe second oxide layer is made of an oxide comprising any one or more of elements Sr, La, Y, Si, and Ge, as main components.5. The light-emitting element according to claim 1, whereinthe second oxide layer has a film thickness of 0.2 nm or more and 5 nm or less.6. The light-emitting element according to claim 1, whereinthe density of the oxygen atoms in the second oxide layer is higher than or equal to 50%, and lower than or equal to 90%, of the density of the oxygen atoms in the first oxide layer.7. A light-emitting device, comprising the light-emitting element according to claim 1.8. Alight-emitting element comprising:a first electrode serving as an anode;a second electrode serving as a cathode;a light-emitting layer provided between the first electrode and the second electrode;a first oxide layer provided between the light-emitting layer and the first electrode; anda second oxide layer provided between the first oxide layer and the light-emitting layer, and having contact with the first oxide layer, whereina density of oxygen atoms in the second oxide layer is lower than a density of oxygen atoms in the first oxide layer,the first oxide layer is made of a semiconductor, andthe first oxide layer comprises at least one of nickel oxide or copper-aluminum oxide.9. The light-emitting element according to claim 8, whereinthe second oxide layer comprises at least one of strontium oxide, lanthanum oxide, yttrium oxide, silicon oxide, germanium oxide, or a complex oxide comprising two or more kinds of cations of the strontium oxide, the lanthanum oxide, the yttrium oxide, the silicon oxide, and the germanium oxide.10. A light-emitting element, comprising:a first electrode serving as an anode;a second electrode serving as a cathode;a light-emitting layer provided between the first electrode and the second electrode;a first oxide layer provided between the light-emitting layer and the second electrode; anda second oxide layer provided between the first oxide layer and the second electrode, and having contact with the first oxide layer, whereina density of oxygen atoms in the second oxide layer is lower than a density of oxygen atoms in the first oxide layer, andthe second oxide layer is made of an n-type semiconductor.11. The light-emitting element according to claim 10, whereinthe second oxide layer is made of an oxide comprising any one of elements In, Sn, or Ti with a highest content other than oxygen.12. The light-emitting element according to claim 10, whereinthe first oxide layer is made of an oxide comprising any one of elements Al, Ga, Ta, Zr, Hf, Ge, or Si with a highest content other than oxygen.13. The light-emitting element according to claim 10, whereinthe second oxide layer is made of zinc oxide, andthe first oxide layer is made of at least one of aluminum oxide, gallium oxide, tantalum oxide, zirconium oxide, hafnium oxide, germanium oxide, silicon oxide, or a complex oxide comprising two or more kinds of cations of the aluminum oxide, the gallium oxide, the tantalum oxide, the zirconium oxide, the hafnium oxide, the magnesium oxide, the germanium oxide, and the silicon oxide.14. The light-emitting element according to claim 10, whereinthe second oxide layer is made of titanium oxide, andthe first oxide layer is made of at least one of aluminum oxide, gallium oxide, or a complex oxide comprising two or more kinds of cations of the aluminum oxide and the gallium oxide.15. The light-emitting element according to claim 10, whereinthe second oxide layer is made of indium oxide, andthe first oxide layer is made of at least one of aluminum oxide, gallium oxide, tantalum oxide, zirconium oxide, hafnium oxide, magnesium oxide, germanium oxide, or a complex oxide comprising two or more kinds of cations of the aluminum oxide, the gallium oxide, the tantalum oxide, the zirconium oxide, the hafnium oxide, the magnesium oxide, and the germanium oxide.16. The light-emitting element according to claim 10, whereinthe second oxide layer is made of tin oxide, andthe first oxide layer is made of at least one of aluminum oxide, gallium oxide, tantalum oxide, or a complex oxide comprising two or more kinds of cations of the aluminum oxide, the gallium oxide, and the tantalum oxide.17. The light-emitting element according to claim 10, whereinthe second oxide layer is made of strontium titanate, andthe first oxide layer is made of at least one of aluminum oxide, gallium oxide, tantalum oxide, zirconium oxide, hafnium oxide, or a complex oxide comprising two or more kinds of cations of the aluminum oxide, the gallium oxide, the tantalum oxide, the zirconium oxide, and the hafnium oxide.18. The light-emitting element according to claim 10, whereinthe density of the oxygen atoms in the second oxide layer is higher than or equal to 50%, and lower than or equal to 75%, of the density of the oxygen atoms in the first oxide layer.19. The light-emitting element according to claim 10, whereinthe first electrode, the second electrode, the light-emitting layer, the first oxide layer, and the second oxide layer are provided above a face of a substrate, anda portion of at least an upper face of either the first oxide layer or the second oxide layer, whichever is positioned below the other one, is polycrystalline.