Fix updating a view z-index on Android

Summary:
If the z-index was updated after the initial mount, changes would not be reflected because we did not recalculate the z-index mapped child views and redraw the view. This adds code to do that and call it whenever we update z-index.

**Test plan**
Tested by reproducing the bug with 2 overlapping views that change z-index every second. Made sure it now works properly and z-index changes are reflected.
Closes https://github.com/facebook/react-native/pull/15203

Differential Revision: D5564832

Pulled By: achen1

fbshipit-source-id: 5b6c20147211ce0b7e8954d60f8614eafe128fb4

ReactAndroid/src/main/java/com/facebook/react/uimanager/BaseViewManager.java

@@ -5,6 +5,7 @@ package com.facebook.react.uimanager;
 import android.graphics.Color;
 import android.os.Build;
 import android.view.View;
+import android.view.ViewParent;
 
 import com.facebook.react.R;
 import com.facebook.react.bridge.ReadableArray;
@@ -80,6 +81,10 @@ public abstract class BaseViewManager<T extends View, C extends LayoutShadowNode
   public void setZIndex(T view, float zIndex) {
     int integerZIndex = Math.round(zIndex);
     ViewGroupManager.setViewZIndex(view, integerZIndex);
+    ViewParent parent = view.getParent();
+    if (parent != null && parent instanceof ReactZIndexedViewGroup) {
+      ((ReactZIndexedViewGroup) parent).updateDrawingOrder();
+    }
   }
 
   @ReactProp(name = PROP_RENDER_TO_HARDWARE_TEXTURE)