Reject incoming call for emergency call

An emergency call fails when receiving a call at the same time.

Case1. Reject incoming call when starting an emergency call.
Case2. Reject incoming call if it receives a new incoming call after
starting an emergency call.

Bug: 328671756
Test: atest EmergencyStateTrackerTest
Manual test:
1. Make a call from A to DUT.
2. Check whether the call is in alerting state in A.
3. At that moment, dial 911 in DUT before the incoming call is notified.

Change-Id: I1bf39b8cad3b0b2709bebdf515d1986913cc653c