Add ANSWERED state to call states

Add an ANSWERED state, indicating that the in-call service has picked up
the call but the connection service hasn't yet marked it active.
Also remove the old audio hack that was used for answered-but-not-active
IMS calls -- it can be gated on the call state now.

Change-Id: Ic91299762354efbf1899a70916d793e4c62ba07e
Fixes: 28349393
Test: manual, unit