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

telecomm/java/android/telecom/Connection.java

@@ -366,6 +366,14 @@ public abstract class Connection extends Conferenceable {
      */
     public static final String EXTRA_CALL_SUBJECT = "android.telecom.extra.CALL_SUBJECT";
 
+    /**
+     * Boolean connection extra key set on a {@link Connection} in
+     * {@link Connection#STATE_RINGING} state to indicate that answering the call will cause the
+     * current active foreground call to be dropped.
+     */
+    public static final String EXTRA_ANSWERING_DROPS_FG_CALL =
+            "android.telecom.extra.ANSWERING_DROPS_FG_CALL";
+
     /**
      * Connection event used to inform Telecom that it should play the on hold tone.  This is used
      * to play a tone when the peer puts the current call on hold.  Sent to Telecom via