AudioService: fix deadlock between readSettings() and VSS.setAllIndexes()

Locking order is mSettingsLock -> VolumeStreamState.class
Missing sync on mSettingsLock was causing deadlock between
  VolumeStreamState.setAllIndexes() and readSettings().
Mark that setAllIndexesToMax() is properly sync'd on mSettingsLock.

Bug: 72122435
Test: chance
Change-Id: I4bd4037fec290dd6ba586c7090e1b377543041ab