Fix bugs for Widgets picker Search UI.

- On orientation change, clear search bar.
- Change text and search bar color to make similar to mocks/ all apps search bar.
- Fix search recycler view visual bugs.
- If back is pressed in search mode, exit search mode instead of exiting widget picker.

Test: Manually seen widget picker on phone.
Bug: 183103133
Bug: 183103936
Bug: 183115050
Bug: 157286785
Change-Id: I056f089eada84d00133038f4a3d7b18bc51be564