Added a DummyConsumer for use with BufferQueue.

The DummyConsumer is a consumer that can connect to BufferQueue
that does nothing.  It is required as BufferQueue checks
if a consumer is connected.  Also fixes a bug where SurfaceTexture
was reusing old texture slots.

Bug: 6172739
Change-Id: I5e7a118dd4d07807ba4c763200242b5ee7d3412b