disconnect ongoing unholdable call for transactional calls

Previously, if an ongoing unholdable call was active and a new
incoming transactional call came in, the notifications answer
button would actually disconnect the transactional call in favor of
keeping the active unholdable call.

Now, CallControlCallback requests for transactional calls can disconnect
the ongoing unholdable calls. This behavior makes things more consistent
with the legacy ConnectionService way.

Fixes: 340621152
Flag: com.android.server.telecom.flags.transactional_hold_disconnects_unholdable
Test:  7 unit tests + manual testing
        (1) start unholdable call
	(2) create incoming transactional call
	(3) answer via notification
	observe/expect: unholdable call disconnects &
	          transactional call becomes active
Change-Id: Id941d7f34454ca31dddf67c121762ee392c790e8