Call AudioMixer only from MixerThread threadLoop.

As part of change:
Remove track name offset by TRACK0.
Move track name management to the Tracks class.
Sync mixer track name to FastMixer track index.

Fixes regression introduced by commit 8ed196ac.

Test: SoundPool, AudioTrack CTS, Usability
Bug: 72937362
Bug: 73004420
Change-Id: I2f1a33f6f0da66bcd7aa91e2a4b663ff822df645