[iOS] Fix disabledBackGesture. (#1281)

* Fix disabledBackGesture.

This `return self._disableBackGesture ? self._disableBackGesture : YES;` check is incorrect, it's the same as `true ? true : true;`.
Also `_disableBackGesture` was never set. We should just `return disabledBackGestureBool` (bool value of the prop).

* [iOS] Enable back swipe gesture when a navBar is hidden. Resolves this issue: https://github.com/wix/react-native-navigation/issues/930

added 03d380f8a9 back and also applied this fix https://github.com/wix/react-native-navigation/issues/428#issuecomment-278268928 on top of it

* Revert accidental commit: 3cdb1b37f6

ios/RCCViewController.m

@@ -24,7 +24,6 @@ const NSInteger TRANSPARENT_NAVBAR_TAG = 78264803;
 @property (nonatomic) BOOL _statusBarHideWithNavBar;
 @property (nonatomic) BOOL _statusBarHidden;
 @property (nonatomic) BOOL _statusBarTextColorSchemeLight;
-@property (nonatomic) BOOL _disableBackGesture;
 @property (nonatomic, strong) NSDictionary *originalNavBarImages;
 @property (nonatomic, strong) UIImageView *navBarHairlineImageView;
 @end
@@ -658,7 +657,9 @@ const NSInteger TRANSPARENT_NAVBAR_TAG = 78264803;
 
 #pragma mark - UIGestureRecognizerDelegate
 -(BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer {
-  return self._disableBackGesture ? self._disableBackGesture : YES;
+  NSNumber *disabledBackGesture = self.navigatorStyle[@"disabledBackGesture"];
+  BOOL disabledBackGestureBool = disabledBackGesture ? [disabledBackGesture boolValue] : NO;
+  return !disabledBackGestureBool;
 }