Announce scaled value for font scaling seekbar in Talkback

1. Keeping state descriptions for the seekbar as an array in the
   PreviewSizeSeekBarController when we would like to customize the
   state description.
2. Setting the scaled value to be the state description for the font
   scaling seekbar when there is a progress change to the seekbar.

Bug: 269212483
Test: manually - attach videos to the bug
Test: make RunSettingsRoboTests ROBOTEST_FILTER=PreviewSizeSeekBarControllerTest
Change-Id: I8e9e3681a0174f454cac871ebf62d785962c7165