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
Steven Ng