libgui: add BQ consumer buffer free notifications

This change adds a new callback for BufferQueue consumers to be notified
when the BufferQueue frees some or all of its buffers.  This is needed
to retain SurfaceTexture behavior where all buffers would be freed when
the producer disconnects.  This change also modifies the
SurfaceTextureGLToGLTest.EglDestroySurfaceUnrefsBuffers test to catch
when the buffers are not freed.

The implementation is a little complicated because it needs to avoid
circular sp<> references across what will be a binder interface (so wp<>
can't be used directly).  It also needs to avoid the possibility of
locking the BufferQueue and consumer (e.g. SurfaceTexture) mutexes in
the wrong order.

This change also includes a few additional fixes and test cleanups.

Change-Id: I27b77d0af15cb4b135f4b63573f634f5f0da2182