Handles blacklist on getRestrictBackgroundStatus().

Initial implementation of getRestrictBackgroundStatus() was just
handling the whitelist status of apps when data saver is turned on, but
when an app is blacklisted, it should return
RESTRICT_BACKGROUND_STATUS_ENABLED regardless of the data saver status.

BUG: 27432317
Change-Id: I5a027d8b7d17a13075510ff8f29d41eaa5a6beb8