Clean up AudioTrack position and timestamp handling

Replace epoch concept by observing and accumulating server delta
positions.  The advantage of using server deltas instead of absolute
values is that they (1) are not sensitive to 32-bit wraparound,
(2) are not sensitive to server behavior for stop(), and
(3) prepare for future 64-bit client positions without requiring 64-bit
positions on server.

Add comments to AudioTrack::getTimestamp() and friends
that the timestamp output parameter is undefined on error.

Don't allow getTimestamp to return a negative frame position after stop().

Accumulate the client released frames, which may be useful for a future API.

Bug: 11815245
Change-Id: I652940fa2db2f34a78c012a3ead0d9204fa29c6e