Refactor SurfaceTexture a bit.

Rearranges updateTexImage() so that the SurfaceFlinger-specific
behavior is in a new SurfaceFlingerConsumer subclass.

SurfaceTexture behavior should not be altered.  Instead of
acquire-bind-release we now do acquire-release-bind, but since
it's all done with the lock held there shouldn't be any
externally-visible change.

Change-Id: Ia566e4727945e2cfb9359fc6d2a8f8af64d7b7b7