Change the font size after interacting with SeekBar

Since it will lead to lags on the screen if we set the font scale to
system everytime the progress changes when we drag through the seekbar,
we change the behavior to be only changing the system font scale when users
release their finger from the seekbar.

Bug: 274395502
Test: Manually - attach videos to the bug
Test: atest FontScalingDialogTest
Change-Id: I3b4168414c93d847f10ce081d1617010f62c2e21