cross-SIM calling: support e911 call on cellular sub

- When in a voice call on backup calling, always place e911 call on
  cellular sub.
- Based on carrier policy, hold or disconnect the existing backup
  calling voice call, just as in single-SIM.

Bug: 261901444
Test: On-device DSDA e911 tests for TMO and ATT, with existing cross-SIM
call; New UTs in TelephonyConnectionServiceTest.

Change-Id: I3809925efcf1cbb15d00a54f98959c7b3c69718e
2 files changed