HFP: Add isInbandRingingEnabled() API (1/4)

* Use BluetoothHeadset.isInbandRingingEnabled() API to check whether
  in-band ringing is currently enabled in the system when deciding on
  audio routes for ringtone
* Add this as a hidden internal API for system components

Bug: 71646213
Test: make, toggle in-band ringing from Development Settings and observe
      whether Telecom service tries to open SCO when there is an
      incoming call

Change-Id: I1ef0dd2b54ace7649ddd1f043f0ef5847743a5c4