Change search fab to search bar.

- Remove search fab from layout xml
- Change RelativeLayout to CoordinatorLayout so we can use prebuilt
  scrolling behavior
- Minor update to theme so search bar background works in dark mode for
  the homepage activity.

Change-Id: If7408c12684be65137e04ae3bb4137204c2d77e0
Fixes: 117508596
Test: robotests, visual