Fix few UI bugs in widget picker.

- Update colors for widget picker (b/184236316)
- Adjust padding according to mocks (b/184236316)
- Make sure view holders refresh corners in search mode. (b/185099541)
- Fix search bar shadow being cropped (b/183602001)

Bug: 184236316
Bug: 185099541
Bug: 183602001
Test: Manually tested
Change-Id: Ifa33e090cdf31c95db9c1145e623ef55c42dd52e