Refactors Search results into separate RV for Toast.

This will help enable transitions between A-Z apps lists and
search results because both can be seen simultaneously and
manipulated independently.

Some high level items of the refactor:
 - SearchRecyclerView is added; logic that populated the main
   (personal) tab with search results was simply redirected to
   this RV instead.
 - BaseAllAppsContainerView added isSearching() method. Returns
   false, and ActivityAllAppsContainerView overrides (as search
   is handled there).
 - Renamed BaseRecyclerView to FastScrollRecyclerView to better
   describe what it does. SearchRecyclerView extends this, but
   returns false for supportsFastScrolling().
 - AlphabeticalAppsList#mAllAppsStore is now optional, so the
   Search RV doesn't need to store/listen to apps. Note this
   doesn't affect the predicted app row which is still updated
   if one of the predicted apps is uninstalled (I tested this).

Future work:
 - Determine why dispatchRestoreInstanceState is not called for
   BaseAllAppsContainerView. Save is called, e.g. on rotation.
   Effect of restore not called: rotating while searching goes
   back to A-Z list.
 - Keep suggested apps in Header while searching. Currently they
   are rendered in the SearchRV above search results, as before.
 - Potentially extract Personal/Work tabs to move independently of
   header.
 - AlphabeticalAppsList is a misleading name because it can also
   contains search results. However, things are pretty intertwined
   between that and BaseAllAppsAdapter (effectively a circular
   dependency), so I figured cleaning all that up was out of the
   immediate scope of this refactor, which is mainly meant to
   unblock transition work.

Bug: 206905515
Test: Manually checked for regressions, ran tests.
Change-Id: I4d3757c8a8f9b774956ca6be541dd4fcdad1de13