Show an empty state when all widgets have been removed

Show a full size CTA for users to add widgets when there is
no widget available in GH.
Added an option in EditWidgetsActivity to automatically open
the widget picker upon entering the edit mode.

Bug: b/330175529
Test: atest CommunalViewModelTest
Test: atest CommunalInteractorTest
Test: manually remove all widgets and verify the flow
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: Iacf4b0f6e8d2580a07265663f40efd3f406d441b