Release buffer slot basing on reference counter

This patch introduces counter which stores number
of references kept by CodecBuffers for each GraphicBuffer
on given slot.
The counter is decreased after codec buffers emptied
and increased after buffers submitted. When counter
becomes 0 it means that given buffer slot shall be
released.

Bug: 30024369

Cherry-picked with (rebase and fix) from:

https://android-review.googlesource.com/#/c/246510/2

Change-Id: I4ecd7902a6a525f9a598682d9cccc0f771752280