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.

Change-Id: I96f80d93e740a25a6a6890b423a41c3900b93767

Conflicts:
	res/values/attrs.xml
	res/xml/display_settings.xml
	src/com/android/settings/DisplaySettings.java