Make some accessibility related slices public.

Only PreferenceControllers with isPublicSlice() set to true are exposed
to other apps. Others will be Settings only.

Bug: 141088937
Test: robotests
Change-Id: If0fa79f7c270b10f8f1a47cb5f914d19566dedfb

src/com/android/settings/accessibility/MagnificationGesturesPreferenceController.java

@@ -69,6 +69,11 @@ public class MagnificationGesturesPreferenceController extends TogglePreferenceC
                 "screen_magnification_gestures_preference_screen");
     }
 
+    @Override
+    public boolean isPublicSlice() {
+        return true;
+    }
+
     @Override
     public CharSequence getSummary() {
         int resId = 0;