Fix the display/Font preview window appears blank while transitioning between sizes.

Action:
Using post messages to update the UI to avoid the timing issue.

Bug: 223665259
Test: make RunSettingsRoboTests ROBOTEST_FILTER=PreviewSizeSeekBarControllerTest
Change-Id: I31ef0d50a417fb76d14c1caf21926f1618ac3c4f

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

@@ -48,7 +48,9 @@ class PreviewSizeSeekBarController extends BasePreferenceController implements
 
                     final ProgressInteractionListener interactionListener =
                             mInteractionListener.get();
-                    interactionListener.notifyPreferenceChanged();
+                    // Avoid timing issues to update the corresponding preview fail when clicking
+                    // the increase/decrease button.
+                    seekBar.post(interactionListener::notifyPreferenceChanged);
 
                     if (!mSeekByTouch) {
                         interactionListener.onProgressChanged();