Refactoring before adding a new view type in the WidgetsListAdapter

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