aaudio: name threads for easier debugging

Name them "AAudio_#" where # is an increasing index.
For example AAudio_1, AAudio_2, etc.

Also name threads that are used for sending timestamps.

Test: run write_sine_callback then dump the threads for that process.
Change-Id: Ibd8c7be1109f6c15a4601ef154038cdce2e65e11