Add subclass for two pane widget picker

Since the regular widget picker and the widget picker for tablet diverge quite a bit, we added a child class for it that extends WidgetsFullSheet.java
There should be no functionality changes with this CL, it's purely changes in code structure.

Bug: 256684299
Test: Turn on LARGE_SCREEN_WIDGET_PICKER flag and verify nothing changes compared to ag/20508230.
Change-Id: Ie0e93534d7b93017cc11a05f67728155d22951f9