Add Device to Device communication carrier configs.

Key configurations required:
- Whether carrier supports using DTMF A-D digit D2D comms.
- Whether carrier supports using RTP header extensions for D2D.
 - If using RTP header extensions, whether SDP negotiation of the
   RTP header extensions is supported or not.

Test: Add unit tests for new carrier config functionality.
Test: Manual test using telephony cmds to enable/disable the various
config options.
Bug: 163085177

Change-Id: I7422b90e655ef190d474cc12d1becee3074f657b