Enable text preview in dialog when dragging through seekbar

We use the heading of font scaling dialog to provide font size preview
when users are dragging through the seekbar and before they release
their fingers from the seekbar.

Bug: 274395502
Test: Manually - attach viideos to the bug
Test: atest FontScalingDialogTest
Change-Id: I3290d8c3560fea86453674bfb1f935ba6f709609