Fix unit test problems in MobileNetworkSummaryController

There were two failing tests due to problems in this class:
-PreferenceControllerContractTest
-SettingsSearchIndexablesProviderTest

In one case it was because we weren't checking for a valid Lifecycle,
and in the other it wasn't implementing PreferenceControllerMixin (or
inheriting from BasePreferenceController).

Bug: 127524919
Test: atest SettingsUnitTests
Change-Id: I662e300d68a21c28e1efb3ec9e23c4a882497176