Add support for dropping fg video call on answer of audio call.

Adding two new carrier config options:
1. treat_downgraded_video_calls_as_video_calls_bool - when there is an ongoing
call which was previously a video call (or is currently video call), some
carriers require that this is treated as if it is a video call for
telephony logic such as in (2).
2. drop_video_call_when_answering_audio_call_bool - if the user is in an
active video call (see (1)) over wifi, and an incoming audio call comes in,
when this carrier config option is "TRUE", the video call will be dropped
when the audio call is answered.

Adding new connection extra to indicate to the dialer app that answering
a ringing call will cause the current active video call to drop.

Bug: 29275420
Change-Id: Ied5dbabc8ff09045ba41c5eba2136adad10db80e