aaudio: fix race condition in timestamp path

There was a potential race between setting the first timestamp
and reading the first timestamp that could cause the timestamp service
thread to exit. This would cause the client to not get any timestamps.
Then the timing model would not advance and the stream would stall.
This could result in read() or write() returning zero,
or a TIMEOUT error.

This was rare except for some devices that had different
timing in the HAL.

Bug: 68271891
Bug: 117557847
Test: adb shell write_sine_callback -pl -m2 -n2 -s2 -c2 -l100
Test: atest CtsNativeMediaAAudioTestCases
Change-Id: Ib0974a576aaa17ff8de40899e60ed56015027bd7