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

* Use BluetoothHeadset.isInbandRingingEnabled() API to check whether
  in-band ringing is currently enabled in the system when deciding on
  audio routes for ringtone

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: I2657102b5239f52d64f2f82d9b45f712c28bdba7