Fix preference version handling

Move preference upgrade of theme to version 4 of the preferences. The
upgrade is meant FROM that version to the next.

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/util/Preferences.java

@@ -390,11 +390,11 @@ public class Preferences {
                 }
                 // fall through
                 case 4: {
-                    // fall through
-                }
-                case 5: {
                     setTheme(Constants.Pref.Theme.DEFAULT);
                 }
+                // fall through
+                case 5: {
+                }
             }
 
             // write new preference version