SurfaceTexture: add context attach & detach

This change adds the detachFromContext and attachToContext methods to
SurfaceTexture.  These methods allow the SurfaceTexture to switch from
one consumer GLES context to another.  This change also includes a few
cleanups to the error return codes in updateTexImage.

Change-Id: I0df1eb599aa7b6f58f07431f242f8f09269559ed