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
diff --git a/partitionmanager.cpp b/partitionmanager.cpp
index 031cca0..a326d79 100644
--- a/partitionmanager.cpp
+++ b/partitionmanager.cpp
@@ -772,7 +772,7 @@
 		return false;
 	}
 
-	DataManager::GetValue("tw_disable_free_space", disable_free_space_check);
+	DataManager::GetValue(TW_DISABLE_FREE_SPACE_VAR, disable_free_space_check);
 
 	if (adbbackup)
 		disable_free_space_check = true;