Add the settings content observer for shortcut preference.

Root cause:
Didn't update the UI when settings value chagned.

Next:
Add the settings content observer.

Bug: 155837154
Test: manual test
Change-Id: Icee0bb50f88987ea6c8a0be59bd936628a32ca1e

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

@@ -467,7 +467,8 @@ public class ToggleScreenMagnificationPreferenceFragment extends
         showDialog(DialogEnums.MAGNIFICATION_EDIT_SHORTCUT);
     }
 
-    private void updateShortcutPreferenceData() {
+    @Override
+    protected void updateShortcutPreferenceData() {
         // Get the user shortcut type from settings provider.
         mUserShortcutType = getUserShortcutTypeFromSettings(getPrefContext());
         if (mUserShortcutType != UserShortcutType.EMPTY) {
@@ -489,7 +490,8 @@ public class ToggleScreenMagnificationPreferenceFragment extends
         mShortcutPreference.setTitle(title);
     }
 
-    private void updateShortcutPreference() {
+    @Override
+    protected void updateShortcutPreference() {
         final int shortcutTypes = getUserShortcutTypes(getPrefContext(), UserShortcutType.SOFTWARE);
         mShortcutPreference.setChecked(
                 hasMagnificationValuesInSettings(getPrefContext(), shortcutTypes));