Add summary for Backup menu in Settings.

'On'/'Off' depending on whether the backup is enabled in settings.

Refactors BackupSettingsActivity to be shown on System fragment via
xml/PreferenceController instead of category in the manifest.

Bug: 36891827
Test: make RunSettingsRoboTests
Change-Id: I61425fed4d63055c6c386be38792ae074aa72679

src/com/android/settings/system/SystemDashboardFragment.java

@@ -21,6 +21,7 @@ import android.provider.SearchIndexableResource;
 
 import com.android.internal.logging.nano.MetricsProto;
 import com.android.settings.R;
+import com.android.settings.backup.BackupSettingsActivityPreferenceController;
 import com.android.settings.core.PreferenceController;
 import com.android.settings.dashboard.DashboardFragment;
 import com.android.settings.deviceinfo.AdditionalSystemUpdatePreferenceController;
@@ -60,6 +61,7 @@ public class SystemDashboardFragment extends DashboardFragment {
         final List<PreferenceController> controllers = new ArrayList<>();
         controllers.add(new SystemUpdatePreferenceController(context, UserManager.get(context)));
         controllers.add(new AdditionalSystemUpdatePreferenceController(context));
+        controllers.add(new BackupSettingsActivityPreferenceController(context));
         return controllers;
     }