Finish creating disable_free_space_check variable

* Create data manager variable for 'tw_disable_free_space'
* Make configurable through Settings
* Fix the "two presses needed to enable" issue for the checkbox on the
  backup screen
* No need to capitalize every word, but do specify precedes backup

Change-Id: Id436cef13e4ca9349618420aac03862ec4e3c35e