Settings: Prevent DataSaverBackend from overwriting uid policies

* Lineage assigns additional network policy values for restrict on
  data and wlan etc.

* DataSaverBackend was assuming that only allow background
  and allow unrestricted background exist and therefore could both:
  i) overwrite other uid policies and ii) incorrectly think there were
  changes when there weren't.

* Change setUidPolicy to appropriate add/remove sequences and filter
  the newPolicy value in the callback to ignore everything except
  background data related policies.

Change-Id: I4c81d7fa2199441674a4176eb78b29facd0b799e