Add title to SeekbarPreference content description.

Adding a title to contentDesciption of the seekbar.
The title is helpful to indicate which seekbar user is changing when
talkback is enabled.

Also move SeekBarPreference from root package to widget packge.

Change-Id: I793f41c4902e350b6bc6d62761362779cc3722a9
Fix: 35421942
Test: make RunSettingsRoboTests