Added a configuration option indicating if the platform supports use of

bluetooth SCO for off call use cases.
Added method to BluetoothHeadset API to check if voice dialer over
bluetooth is supported. Replaces direct use of DISABLE_BT_VOICE_DIALING
and takes platform capabilities into account.

Needed for issue 2416481: Support Voice Dialer over BT SCO.

Change-Id: I237d5a984dbc4cbc88e98b85d710a6c87ba1b315