Expose C2BufferQueueBlockPoolData definition at header file

C2BufferQueueBlockPoolData class is used to track the ownership of
C2GraphicBlock and migrate the buffer when surface switching. It's
originally only accessed by C2BufferQueueBlockPool and
_C2BlockFactory, so we hide the definition at cpp file.

However, this class is highly coupled with the CCodec buffer
transition logic. Without exposing the definition, the child class of
C2BufferQueueBlockPool (i.e. C2VdaBqBlockPool) has to add hacky
workaround for the same logic. Furthermore, C2VdaBqBlockPool cannot
migrate all the buffers when surface switching.

This CL exposes the definition of C2BufferQueueBlockPoolData to the
header file, and let C2VdaBqBlockPool could access the class.

Bug: 174188958
Test: android.media.cts.AdaptivePlaybackTest

Change-Id: Ic0d76e3bd4d3e14ea6023b0c1b0fac662028dbe7