Display Settings: Better font size control

Use a SeekBar instead of a list/spinner to determine the font scaling.
For consistency, an approximate size (small, normal, etc.)
 and the exact percentage are displayed.
Show a preview of the selected font size immediately
Split the size description and percentage on their own lines to show
 a preview of two different font sizes.

This is a combination of two commits merged into CM:
http://review.cyanogenmod.org/#/c/46678 and
http://review.cyanogenmod.org/#/c/49167/

  Patch Set 2: Remove unnecessary dependency

Change-Id: Ia9f325797ccfbf6d7ad4cf958b65ef1ee51c2342