create junit MobileNetworkPreferenceControllerTest.

1. androidx lifecycle used rather than settingslib wrapper. This allows
us to handle lifecycle events from junit tests as well.
2. Use Mocks or get/set functions rather than robolectric shadows.

MobileNetworkPreferenceControllerTest Diff:
    https://diff.googleplex.com/#key=5P0b2G4iYmcz
Test: atest -c MobileNetworkPreferenceControllerTest
Change-Id: If49fe64e5db89a0cdcac7d26329b8c18626a00de