AlarmManager: Store time offset system property

For TILB (time in low battery mode) we need to store the time zone
offset as a system property. Updating AlarmManagerService so it matches
kr3-dev and stores this offset when the time zone is updated through
connectivity or when it is updated manually in the settings.

Bug: 311191984
Test: Update time zone manually and check the system property
Change-Id: I426941c01408bb83821573f83e757c05aec4dd5c