Create and plumb SkiaBackendTexture abstraction layer over GrBackendTexture

This means AutoBackendTexture is not aware of backend-specific texture
type and API details, and will be able to accept either a Ganesh or
Graphite variant of SkiaBackendTexture.

Also delegated SkiaBackendTexture creation to SkiaGpuContext, so that
backend-specific contexts handle creating backend-specifc textures.

Test: manual validation (GL+VK) & existing tests (refactor)
Bug: b/293371537
Change-Id: Ia65306cc825b71fe0b89c7f8545ce1c71a81d86b