C2VdaBqBlockPool: make callback members hold weak_ptr of C2VdaBqBlockPool

BufferReleasedNotifier and C2VdaBqBlockPoolData notify
C2VdaBqBlockPool when some event happens. Originally these two classes
hold shared_ptr of C2VdaBqBlockPool. However they should not hold the
ownership of C2VdaBqBlockPool. This CL changes these two classes to
hold weak_ptr of C2VdaBqBlockPool.

Bug: 160110407
Test: android.media.cts.AdaptivePlaybackTest

Change-Id: Ibc8c960dabc16542f83f5e6ff082dec70edf2481