aaudio: fix timestamps and underruns

Start the client after we get valid timing data from the server.
That can take a while because of the long cold start times.
The client is synced with the current position of the service.
Now the client can start clean with no underruns.

Bug: 63918065
Test: test_timestamps.cpp
Change-Id: I5d01eb844e4b14cd5477d56ea1dd9e309abc1c52