Introduce system APIs for Bluetooth silence mode.

Policies of silence mode:
1) If an active device (for A2DP or HFP) enters silence mode,
   the active device for that profile will be set to null.
2) If a device exits silence mode while the A2DP or HFP active
   device is null, the device will be set as the active device
   for that profile.
3) If a device is disconnected, it exits silence mode.
4) If a device is set as the active device for A2DP or HFP,
   while silence mode is enabled, then the device will exit
   silence mode.
5) If a device is in silence mode, AVRCP position change event
   and HFP AG indicators will be disabled.
6) If a device is not connected with A2DP or HFP, it cannot
   enter silence mode.

Bug: 112323989
Test: Manual, runtest bluetooth
Change-Id: If7f340de38c350f17c37f00a3a2e871876baa20d