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();

tests/robotests/src/com/android/settings/accessibility/PreviewSizeSeekBarControllerTest.java

@@ -24,6 +24,7 @@ import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
 import android.content.Context;
+import android.widget.SeekBar;
 
 import androidx.preference.PreferenceScreen;
 import androidx.test.core.app.ApplicationProvider;
@@ -111,4 +112,15 @@ public class PreviewSizeSeekBarControllerTest {
 
         verify(mInteractionListener).onProgressChanged();
     }
+
+    @Test
+    public void onProgressChanged_verifyNotifyPreferenceChanged() {
+        mSeekBarController.displayPreference(mPreferenceScreen);
+
+        mSeekBarPreference.setProgress(mSeekBarPreference.getMax());
+        mSeekBarPreference.onProgressChanged(new SeekBar(mContext), /* progress= */
+                0, /* fromUser= */ false);
+
+        verify(mInteractionListener).notifyPreferenceChanged();
+    }
 }