Added overrideBackPress to LightBox (#1520)

android/app/src/main/java/com/reactnativenavigation/params/LightBoxParams.java

     public NavigationParams navigationParams;
     public StyleParams.Color backgroundColor;
     public boolean tapBackgroundToDismiss;
+    public boolean overrideBackPress;
 }

android/app/src/main/java/com/reactnativenavigation/params/parsers/LightBoxParamsParser.java

         result.navigationParams = new NavigationParams(params.getBundle("navigationParams"));
         result.backgroundColor = getColor(params, "backgroundColor");
         result.tapBackgroundToDismiss = params.getBoolean("tapBackgroundToDismiss");
+        result.overrideBackPress = params.getBoolean("overrideBackPress");
         return result;
     }
 }

android/app/src/main/java/com/reactnativenavigation/views/LightBox.java

         setOnDismissListener(this);
         requestWindowFeature(Window.FEATURE_NO_TITLE);
         createContent(activity, params);
+        setCancelable(!params.overrideBackPress);
         getWindow().setWindowAnimations(android.R.style.Animation);
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
             getWindow().addFlags(WindowManager.LayoutParams.FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS);