BufferQueue/SF: Add OccupancyTracker

Adds an OccupancyTracker to BufferQueue. This module keeps track of
how many buffers are in the queue over time, which, in combination
with various aggregation of these statistics, allows SurfaceFlinger
to report what fraction of the time a given layer was double- or
triple-buffered.

Change-Id: Ida6e967dc5483c00a633e9fe03998e420dd88502