Revert "RingBufferConsumer - Remove unnecessary grab"

This reverts commit fb5a3b76.

Reason for revert: caused tests to break b/390301197

ConsumerBase classes are required to set the graphic buffer in the
buffer item for things to work. This is, of course, unfortunate, but we
don't want people to be subclassing this anyway.

Change-Id: I948845b234b170a4942c7d463a2d03f9251f4a02