Improve search bar UI in All Apps

This is similar to, but not exactly the same as, Pixel stock:

- Background color with contrast in light mode
- Round, flat search bar surface with color matching the header
  background
- Solid rounded background when positioned at the beginning of the list
- Subtle placeholder text (medium weight) without icon
- Same font size as Settings search bar

Change-Id: Ifa9fabe3a8236513fad8030c0bd8ed4d27ebd549