Prefer use of view controller hierarchy to display lightbox (#1439)

This fixes issues with other React Native views which rely on the view controller hierarchy.

ios/RCCLightBox.m

 #import <React/RCTRootView.h>
 #import <React/RCTRootViewDelegate.h>
 #import <React/RCTConvert.h>
+#import <React/RCTUtils.h>
 #import "RCTHelpers.h"
 #import <objc/runtime.h>
 
 
 @implementation RCCLightBox
 
-+(UIWindow*)getWindow
-{
-    UIApplication *app = [UIApplication sharedApplication];
-    UIWindow *window = (app.keyWindow != nil) ? app.keyWindow : app.windows[0];
-    return window;
-}
-
 +(void)showWithParams:(NSDictionary*)params
 {
-    UIWindow *window = [RCCLightBox getWindow];
-    if ([window viewWithTag:kLightBoxTag] != nil)
+    UIViewController *viewController = RCTPresentedViewController();
+    if ([viewController.view viewWithTag:kLightBoxTag] != nil)
     {
         return;
     }
-    
+
     RCCLightBoxView *lightBox = [[RCCLightBoxView alloc] initWithFrame:CGRectMake(0, 0, [UIScreen mainScreen].bounds.size.width, [UIScreen mainScreen].bounds.size.height) params:params];
     lightBox.tag = kLightBoxTag;
-    [window addSubview:lightBox];
+    [viewController.view addSubview:lightBox];
     [lightBox showAnimated];
 }
 
 +(void)dismiss
 {
-    UIWindow *window = [RCCLightBox getWindow];
-    RCCLightBoxView *lightBox = [window viewWithTag:kLightBoxTag];
+    UIViewController *viewController = RCTPresentedViewController();
+    RCCLightBoxView *lightBox = [viewController.view viewWithTag:kLightBoxTag];
     if (lightBox != nil)
     {
         [lightBox dismissAnimated];