LatinIME: Remove dividers everywhere

* In order to make it look less like it doesn't
  belong to settings, remove the divider lines that
  are currently everywhere
* Instead of removing the dividers per fragment,
  create a custom fragment which does the job

Test:
* Go to every setting and verify that there are no
  divider lines between every preference

Change-Id: Id76dfa2e3ce3999242c8dd09a531ab06ccacd203