[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

src/com/android/settings/backup/BackupSettingsHelper.java

@@ -209,7 +209,7 @@ public class BackupSettingsHelper {
     }
 
     /** Checks if backup service is enabled for this user. */
-    private boolean isBackupServiceActive() {
+    public boolean isBackupServiceActive() {
         boolean backupOkay;
         try {
             backupOkay = mBackupManager.isBackupServiceActive(UserHandle.myUserId());