Migrate AlarmManager to DeviceConfig.

This migrates flags inside AlarmManagerService to DeviceConfig.
DeviceConfig is the recommended way and using it means
that we only operate/parse the set of flags that changed instead of
every single flag whenever one of them changes.

Bug: 124466289
Bug: 159068048
Test: atest AlarmManagerServiceTest
Test: atest CtsAlarmManagerTestCases
Test: atest CtsBatterySavingTestCases:BatterySaverAlarmTest
Test: atest SettingsProviderTest:SettingsBackupTest
Test: atest com.android.server.cts.IncidentdTest
Change-Id: I21263f54f57bf2a59daafbc35fe6bdfbaf4f031b