SurfaceTexture: inherit from ConsumerBase (try 2)

This change makes SurfaceTexture inherit from ConsumerBase.  It removes all of
the functionality from SurfaceTexture that is now provided by the base class.
This includes fixes for two bugs that were found after checking this change in
the first time and then reverting it.

Change-Id: Ie2d9f4f27cfef26fdac341de3152e842b01a58d2