DSDA: Reject outgoing call when ringing call present

Make sure we reject the outgoing call when there's a unanswered incoming
call. The user should see a message indicating that they cannot place a
call while the incoming call is unanswered.

Bug: 393957944
Test: atest CallSequencingTests
Test: Manual verification
Flag: com.android.server.telecom.flags.enable_call_sequencing
Change-Id: I0580c68a975f55678294174acc2faa65d6a54c10