Have a recycler view to show search results.

-Have a recycler view layout for search widgets list.
-Make WidgetsFullSheet implement interface- SearchModeListener to get notified when user is using search and also when search results are ready.
-Have a WidgetsListSearchHeaderViewHolderBinder for search result headers which shows subtext in header as concatenated string of widget/shortcut labels.
-Modify WidgetsListAdapter and WidgetsDiffReporter to work well with search recycler view.

Test: Tested prototype locally. Also added robolectric test.
Bug: b/157286785
Change-Id: Ie29d9f295fddb6d727b5fc26a360f514f2f4a763