Implement changes of buffer state in Gain/Post/Acquire/Release methods in BufferHubBuffer.

Create shortcuts to atomics in BufferHubBuffer;
Implement changes of buffer state in the four usages of the buffer;
Create 16 unit tests for the 16 buffer state transitions.

Test: BufferHubBuffer_test
Bug: 118718711
Change-Id: I2067cd141611e66732e28344f26d73f261072b8b