Enable scrolling in screen flash color selector dialog

Screen flash color selector dialog doesn't display correctly in
landscappe mode when adjust to large display size from 'Accessibility >
Display size and text > Display size'. Make the layout be scrollable to
prevent strange clipped UI.

Bug:287671988
Test: check the UI manually
Change-Id: I37eb604d362fe4373d26f97a9397230babc6eafd