Stagefright: push blank buffers when tearing down

This change makes OMXCodec push RGB 565 buffers filled with black to an
ANativeWindow when tearing down after decoding to protected gralloc
buffers.  This allows the OMX tear down to zero out any protected
buffers that were used without the possibility that the buffer is still
being used by SurfaceFlinger or HWComposer.

Bug: 5483222
Change-Id: I8acedd81a7bb67dfdc2fd15733e3375b6ce8d560