Process updates to ringer and audio sequentially

Without this, synchronous calls to postUpdateRingerAndAudio with
different values for shouldApplyToRinger will call the first update to
be missed causing ringer external state to not update

Fixes: 345247710
Test: unit test, manual flash and check
Flag: EXEMPT trivial bug fix
Change-Id: I88386737b5d0983eba240e28cd39efd83118d382