Notification muting conditional to exclusive audio focus request

Summary of feature:
Do not mute notifications when speech recognition recording is
  active, but when an app has requested audio focus with
  AudioManager.AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE.

Implementation:
Move definition of AUDIOFOCUS_NONE to AudioManager where other
  audio focus codes are defined.
Add support for querying the current audio focus type.
When audio focus is requested as GAIN_TRANSIENT_EXCLUSIVE, make
  the corresponding loss by LOSS_TRANSIENT.
Before playing a notification, check whether GAIN_TRANSIENT_EXCLUSIVE
  has been requested.

Bug 8251963

Change-Id: I41edc77326b70639d2fdcb4642c53109995b72a8