Declare "searchable" attribute for preferences.

Now we can easily mark a preference nonIndexable in xml instead of
adding key into searchIndexProvider.

Bug: 112608186
Test: robotests
Change-Id: I0ff16d44bb7b6ad148d3d35f09ca0da0163f73f4

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

@@ -40,7 +40,7 @@ public class BackupSettingsActivityPreferenceController extends BasePreferenceCo
     @Override
     public int getAvailabilityStatus() {
         return mUm.isAdminUser()
-                ? AVAILABLE
+                ? AVAILABLE_UNSEARCHABLE
                 : UNSUPPORTED_ON_DEVICE;
     }