Location subscription persistence update

Location subscriptions are stored in the service's sqlite database.
The service now clears out location subscriptions when it first
starts to ensure that they are not carried over if the VM restarts.
Location subscriptions are not cleared out on scheduled service
restarts (i.e. if the service crashes).

Change-Id: I8ecaeb6128f22478d6295a84d2c35a39336b45e0