aaudio_loopback: use WAV files, reject low gain

Save recording in a WAV file instead of a RAW file.
Add a test mode that reads a WAV file, for debugging auto-correlation.
Cleanup printed reports.
Discard more input buffers at beginning.

Test: this is a test
Change-Id: Ib5ea63882eda325b02716a448897007a945f7312