ThemePicker: Fixes to improve layout

* In order to bring the layout inline with the looks of the 18.1 branch
  we need to modify a few files with content from commits that don't
  apply without changing too much of the code in both ThemePicker and
  WallpaperPicker2
* Therefore define missing dimensions and apply parts of code we need
  to make it look good
* Make the header single line in the preview so a name gets cut instead
  of shifting the whole content of the tile down
* Give the preview_top_bar a margin so in the preview page it's not
  glued to the top
* Change the background of the naming page to primaryColor as well
  -> Add the same divider as on other pages as well
* Don't forget the landscape layouts!

Change-Id: I4f8ae1968d6361f54fd71f4d00fc0d706f21e124