Move GrTextureProvider to src

Move GrTextureProvider to src

With this hidden, we can make further simplification. Just want to test
this change against our external clients first, to make sure we're okay
to proceed.

BUG=skia:

Change-Id: I47f8c8f2912201c2890bc2f9a9d68aa92649a2d4
Reviewed-on: https://skia-review.googlesource.com/9072
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>