Fix Caption preview SubtitleView textSize cannot update correctly

Root caue: When received #OnLayoutChangeListener callback, the mPainText size keeps "zero" after called setTextSize() because the requestLayout() does not trigger the whole view re-measure & re-layout in not ui thread.
Solution: Post #refreshPreviewText to ui thread to get correct bounds to
measure & layout.

Bug: 223696269
Bug: 211178478
Test: Manually test
Change-Id: Idcbeba52876171e5f3641eafc77d203764c78788