Add sideband streams to BufferQueue and related classes

Sideband streams are essentially a device-specific buffer queue that
bypasses the BufferQueue system. They can be used for situations with
hard real-time requirements like high-quality TV and video playback
with A/V sync. A handle to the stream is provided by the source HAL,
and attached to a BufferQueue. The sink HAL can read buffers via the
stream handle rather than acquiring individual buffers from the
BufferQueue.

Change-Id: Ib3f262eddfc520f4bbe3d9b91753ed7dd09d3a9b