Handle blacklisted apps on Data Saver whitelist.

When user blacklists an app for background data access, the
"Unrestricted data access" list should not display the whitelist toggle
option, but rather explain background access is turned off and offer the
option to open the app's data usage screen to turn it back on.

It still does not handle changed received by DataSaverBackend, so the UI
will be stale when the user removes a blacklist and navigates back to
the list.

BUG: 27481520

Change-Id: I9d0fa9f0180a69ce9bd1417921bf89ec3ba31f01