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- Adds a single buffer mode to BufferQueue. In this mode designate the
  first dequeued buffer as the shared buffer. All calls to dequeue()
  and acquire() will then return the shared buffer, allowing the
  producer and consumer to share it.
- Modify the buffer slot state tracking. Add a new SHARED state for
  the shared buffer in single buffer mode. Also track how many times
  a buffer has been dequeued/queued/acquired as it's possible for a
  shared buffer to be both dequeued and acquired at the same time, or
  dequeued/acquired multiple times. This tracking is needed to know
  when to drop the buffer out of the SHARED state after single buffer
  mode has been disabled.
- Add plumbing for enabling/disabling single buffer mode from Surface.

Bug 24940410

Change-Id: I3fc550c74bacb5523c049a227111356257386853