Have Telecom service notify AudioService of RTT call state.

Add AudioManager#setRttEnabled() so that Telecom service can call it
when RTT call state has changed.

Bug: 147037345
Test: Force installed Gboard 8.3.6 and verified voice typing works in
  simulated RTT call and simulated RTT upgrade call.
Change-Id: I1ea3dccdfed145e55b1b1cd516b40a33ac7984d9