Make all widgets collapsed in the full widget picker by default

Changes:
1. Add a WidgetListHeader view for showing icon, app name and a subtitle.
2. Only WidgetListHeaders are always visible to users in the full widget
   picker.
3. Only one widgets list from an app is visible in the full widget picker
   at any one time.

Test: Auto: run add robolectric tests under widget/picker
      Manual: Open full widgets picker. Then, expand and collapse apps.
      Video: https://drive.google.com/file/d/1gzfeEm5IOAu0qHsO77OTS2eMfU7CHJiL/view?usp=sharing

Bug: 179797520
Change-Id: Idac58be23dfeafcb79b3c61b4972d3addb462de1