fifo: add new APIs, and other cleanup

New APIs:
 - buffer()
 - capacity()
 - flush()
 - frameSize()
 - totalFlushed()
 - totalLost()
 - totalReleased()

Pull up mFifo from writer and reader into the base provider class.

Update documentation.

Test: see unit tests in tests/ directory
Change-Id: I1957638c69e1c1dc742b6f024156def757736867
2 files changed