codec2: clean up old block pools

CCodecBufferChannel may create many block pools from a single component,
but only the most recently created block pool is actually used. When
creating a new pool, if there was a previous block pool, make sure it
gets cleaned up, so that the component doesn't end up accumulating block
pools until it is reset or released.

Bug: 176949973
Test: DecodeAccuracyTest + AdaptivePlaybackTest

Change-Id: I0d31c81692687f04bdd9617d508d57448dff1d8d
(cherry picked from commit c162e2b5c76ac465e927c582bc3efe6177841c36)