HFP: Add isInbandRingingEnabled() API (2/4)

* Use BluetoothHeadset.isInbandRingingEnabled() API to check whether
  in-band ringing is currently enabled in the system when deciding on
  audio routes for ringtone
* Implement this API in Bluetooth.apk by checking both:
  1. Whether in-band ringing is supported on this platform
  2. Whether in-band ringing is enabled by system properties
* The actual implementation may change in the future to reflect the
  actual state of in-band ringing as the HFP stack evolves

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: I8e41993632e0f42b5c65df7037d3eae4586b062f