Fix some bugs in widget pickers.

- Update color of Educational tip view contents. Background colorAccentSecondary
in NexusLauncher above S, colorAccent otherwise (b/185834169)
- Update widget picker search bar color and its text font and color. (b/186423026)
- Update button padding and text in pin widget picker. (b/182994384, b/183601584)

Bug: 185834169
Bug: 186423026
Bug: 182994384
Bug: 183601584
Test: Manually tested
Change-Id: I36cd3ed2285677d0c164db8f57f6c90a5f247982