Camera: Free buffers more aggressively

This change attempts to free buffers managerd by
BufferManager more aggresively to reduce memory pressure.

Also fix a small buffer accounting issue: check detachBuffer
actually returns a non-null buffer.

Test: keep taking single shot in GCA, CTS
Bug: 38483630
Change-Id: I6c64d1dc2244cec4f1300bbf3992f66f2167eed2