Fixing the possibility of opening multiple WidgetsFullSheet

The error is that you can open multiple times the view for
selecting widgets for the Home Screen (i.e. WidgetsFullSheet.java)
but only when using a keyboard by using the command ctrl+w
creating a new instance every time you use it. To fix this
I added a singleton pattern so that only one instance of
WidgetsFullSheet.java can be open at any given point.

Test: Manually Tested
Fix: 196704061
Change-Id: I7a364ee5e746e77455da326a21aff6830ac18e8c