Simplify SkImageCacherator slightly

Simplify SkImageCacherator slightly

- lockAsBitmap supported reading back textures to a bitmap, but this was
  only used by one GM. Removed all of that code, and merged the two bitmap
  functions together.
- To make the GM cleaner, don't use SkImageCacherator directly - construct
  actual images.

Bug: skia:
Change-Id: Iad17184a02a72f89ccc851fa158764d6c871befa
Reviewed-on: https://skia-review.googlesource.com/14192
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>