[Multi-user] Show Backup only if Backup is activated

This almost reverts the change to SettingsActivity#doUpdateTilesList in ag/6140694
except that instead of setting enabled to true, we set it to whether
backup is activated for this user.

Bug: 124114084

Test: 1a) Set up secondary user
1b) See that there is no Backup option in Settings Homepage - System
1c) adb shell bmgr --user 10 activate true
1d) Backup option is now present
1e) adb shell bmgr --user 10 activate false
1f) Backup option isnt there again

Change-Id: I3f17aabee92bedc91386444c2f0aede696d0fd13