Fix Font Size Dialog user settings in second/guest user

Get/put settings value from current user in Font Size Dialog to ensure
that we are always read/modify font scale from/for current user.

Bug: 284084419
Test: manually - testing Font Size tile on Second user and Guest
Test: atest FontScalingDialogTest FontScalingTileTest
Change-Id: I11be9d8ece0d6a65b270cb209c6462610874fad1