Add a WidgetsSearchBar(Launcher3) and a WidgetsSearchController.

- Make WidgetsSearchBar in Launcher3 initialize WidgetsSearchController with SimpleWidgetsSearchPipeline
- Modify SimpleWidgetsSearchPipeline to filter widgets entries on widgets/shortcut labels also.

Test: Tested prototype locally. Also added robolectric test.
Bug: b/157286785
Change-Id: I65f5fa0240ffb6d22023167e4e86d94d83bbd9f7