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
Changes made:
1. Model: added an abstract class for storing common information for
entries shown in the full page widgets picker.
2. Introduced a ViewHolderBinder interface to split the logic of binding
data to ViewHolder into separate classes.
3. Move the view holder binding of WidgetsListRow from WidgetListAdapter
to its new class.
4. Move some widgets picker classes into a new picker package.
Test: Auto: Run WidgetsListAdapterTest, WidgetsListRowEntryTest and
WidgetsListRowViewHolderBinderTest.
Manual: open the all apps widgets tray and navigate the list.
Bug: 179797520
Change-Id: Iab29557842bb79156cad84d00a4c5d0db0c5aa06
