Handle prioritized car mode in Telecom.

Add basic support for receiving the broadcast that the prioritized car
mode has changed in Telecom; rest of implementation TBD.
Also added a fancy new car mode dialer app which can be used in car mode.

Bug: 136109592
Test: Use new test app to verify enter/exit car mode with priority.
Test: Run new CTS tests.
Change-Id: I93daadf341e7c158260a1cc76efc6085a29f7a38
Merged-In: I93daadf341e7c158260a1cc76efc6085a29f7a38