BufferItemConsumer: Add onBufferFreedListener

This callback can reliably notify its listener when a GraphicBuffer is
being freed at the consumer end of a BufferQueue. This is extremely
useful when user of BufferItemConsumer caches the GraphicBuffer, so they
can listen to this callback and clean up resource properly.

Bug: 35114769
Test: Built system, flash marlin (should have no functional change as
the listener is optional).
Change-Id: I591cfd3ece697b1b4fb3efbeff987c7960422d24