aaudio: fix loopback test, track magnitude drift

Loopback test was still reporting some glitches.
These were false alarms due to:
  * starting input before output causing an overflow
  * not waiting for the input to reach equilibrium with the output
  * detecting slow drift in magnitude as a glitch

I changed the logic for setting up a test
and track slow changes in magnitude.
Now the rare glitch seems to be a real glitch.

Bug: 78139448
Test: this is a test
Change-Id: Id16d7dc59c0f2e040fe1dfb0337e9ed987d4fd02