Add MockModemManager to support useful helper methods

Bug: 213973498
Test: atest TelephonyManagerTestOnMockModem -c
Change-Id: Ia636327d9658b3524e2944f96c5f9f276008e9e6
Merged-In: Ia636327d9658b3524e2944f96c5f9f276008e9e6
8 files changed