Introduce MockModemTestBase class to remove duplications

Bug: 347750328
Test: atest SatelliteManagerTestOnMockService ConnectivityManagerTestOnMockModem TelephonyManagerTestOnMockModem
Flag: EXEMPT test case refactoring
Change-Id: Ia449f96c78af99d1254ce3d4fc15431deb03d54b
Merged-In: Ia449f96c78af99d1254ce3d4fc15431deb03d54b
(cherry picked from commit 3e5ddf7c91414f6e522aa21a160b3899f902ca6a)
3 files changed