HFP: Allow SCO audio to be forcibly connected

* Normally, Android only allows SCO audio to be connected in several
  allowed cases (e.g. in call, in ringing, in virtual call state, etc)
* Sometimes, it is necessary to force a SCO audio connection (e.g.
  during PTS test)
* This change adds setForceScoAudio(boolean) hidden, system only method
  to allow such behaviour

Bug: 38040125
Test: PTS HFP tests
Change-Id: Iab6bca8ec437d04804d61c169f570c1966d164c1