BufferPool: limit number of idle buffers in caches

Some systems have limits on the total number of graphics buffers that
can be allocated, not just the total size. When dealing with
particularly small buffers, the caches could end up retaining 1000s of
buffers.  This change adds an additional eviction trigger based on the
number of idle cached buffers.

Bug: 170702290
Bug: 171553040
Test: android.video.cts.VideoEncoderDecoderTest on ARCVM
Change-Id: If3f4433ba1794ccb624b864a56619c8613a315f2
(cherry picked from commit 0f50e523ef1404f9afbaa7b919e2801e5d94012a)
Merged-In: If3f4433ba1794ccb624b864a56619c8613a315f2
(cherry picked from commit d620a390)