Add zIndex support

Summary:
Adds zIndex support :)

**Test Plan**

Tested the following components by adding two of each, overlapping them, and setting a high zIndex on the first of the two:

ActivityIndicator
Image
MapView
Picker
ScrollView
Slider
Switch
Text
TextInput
View
WebView

Tested on Android 4.1 and iOS 8.4. Also tested updating zIndexes on Views in my own app.

<img width="359" alt="ios activityindicator" src="https://cloud.githubusercontent.com/assets/4349082/15633473/88f842cc-257b-11e6-8539-c41c0b179f80.png">
<img width="330" alt="android activityindicator" src="https://cloud.githubusercontent.com/assets/4349082/15633475/88f95784-257b-11e6-80c0-2bf3ed836503.png">
<img width="357" alt="ios image" src="https://cloud.githubusercontent.com/assets/4349082/15633474/88f93d80-257b-11e6-9e54-4ff8e4d25f71.png">
<img width="340" alt="android image" src="https://cloud.githubusercontent.com/assets/4349082/15633478/88fd2788-257b-11e6-8c80-29078e65e808.png">
<img width="342" alt="android picker" src="ht
Closes #7825

Differential Revision: D3469374

Pulled By: lexs

fbshipit-source-id: b2b74b71d968ebf73ecfd457ace3f35f8f7c7658

Examples/UIExplorer/ViewExample.js

@@ -93,7 +93,7 @@ var ZIndexExample = React.createClass({
       flipped: false
     };
   },
-  
+
   render() {
     const indices = this.state.flipped ? [-1, 0, 1, 2] : [2, 1, 0, -1];
     return (
@@ -128,7 +128,7 @@ var ZIndexExample = React.createClass({
       </TouchableWithoutFeedback>
     );
   },
-  
+
   _handlePress() {
     this.setState({flipped: !this.state.flipped});
   }

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

@@ -5,6 +5,7 @@
 import android.graphics.Color;
 import android.os.Build;
 import android.view.View;
+import android.view.ViewGroup;
 
 import com.facebook.react.bridge.ReadableArray;
 import com.facebook.react.bridge.ReadableMap;
@@ -21,6 +22,7 @@ public abstract class BaseViewManager<T extends View, C extends LayoutShadowNode
   private static final String PROP_TRANSFORM = "transform";
   private static final String PROP_OPACITY = "opacity";
   private static final String PROP_ELEVATION = "elevation";
+  private static final String PROP_Z_INDEX = "zIndex";
   private static final String PROP_RENDER_TO_HARDWARE_TEXTURE = "renderToHardwareTextureAndroid";
   private static final String PROP_ACCESSIBILITY_LABEL = "accessibilityLabel";
   private static final String PROP_ACCESSIBILITY_COMPONENT_TYPE = "accessibilityComponentType";
@@ -70,6 +72,12 @@ public void setElevation(T view, float elevation) {
     // Do nothing on API < 21
   }
 
+  @ReactProp(name = PROP_Z_INDEX)
+  public void setZIndex(T view, float zIndex) {
+    int integerZIndex = Math.round(zIndex);
+    ViewGroupManager.setViewZIndex(view, integerZIndex);
+  }
+
   @ReactProp(name = PROP_RENDER_TO_HARDWARE_TEXTURE)
   public void setRenderToHardwareTexture(T view, boolean useHWTexture) {
     view.setLayerType(useHWTexture ? View.LAYER_TYPE_HARDWARE : View.LAYER_TYPE_NONE, null);

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

@@ -12,12 +12,20 @@
 import android.view.View;
 import android.view.ViewGroup;
 
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.WeakHashMap;
+
 /**
  * Class providing children management API for view managers of classes extending ViewGroup.
  */
 public abstract class ViewGroupManager <T extends ViewGroup>
     extends BaseViewManager<T, LayoutShadowNode> {
 
+  public static WeakHashMap<View, Integer> mZIndexHash = new WeakHashMap<>();
+
   @Override
   public LayoutShadowNode createShadowNodeInstance() {
     return new LayoutShadowNode();
@@ -34,6 +42,59 @@ public void updateExtraData(T root, Object extraData) {
 
   public void addView(T parent, View child, int index) {
     parent.addView(child, index);
+    reorderChildrenByZIndex(parent);
+  }
+
+  public static void setViewZIndex(View view, int zIndex) {
+    mZIndexHash.put(view, zIndex);
+    // zIndex prop gets set BEFORE the view is added, so parent may be null.
+    ViewGroup parent = (ViewGroup) view.getParent();
+    if (parent != null) {
+      reorderChildrenByZIndex(parent);
+    }
+  }
+
+  public static void reorderChildrenByZIndex(ViewGroup view) {
+    // Optimization: loop through the zIndexHash to test if there are any non-zero zIndexes
+    // If there aren't any, we can just return out
+    Collection<Integer> zIndexes = mZIndexHash.values();
+    boolean containsZIndexedElement = false;
+    for (Integer zIndex : zIndexes) {
+      if (zIndex != 0) {
+        containsZIndexedElement = true;
+        break;
+      }
+    }
+    if (!containsZIndexedElement) {
+      return;
+    }
+
+    // Add all children to a sortable ArrayList
+    ArrayList<View> viewsToSort = new ArrayList<>();
+    for (int i = 0; i < view.getChildCount(); i++) {
+      viewsToSort.add(view.getChildAt(i));
+    }
+    // Sort the views by zIndex
+    Collections.sort(viewsToSort, new Comparator<View>() {
+      @Override
+      public int compare(View view1, View view2) {
+        Integer view1ZIndex = mZIndexHash.get(view1);
+        if (view1ZIndex == null) {
+          view1ZIndex = 0;
+        }
+
+        Integer view2ZIndex = mZIndexHash.get(view2);
+        if (view2ZIndex == null) {
+          view2ZIndex = 0;
+        }
+        return view1ZIndex - view2ZIndex;
+      }
+    });
+    // Call .bringToFront on the sorted list of views
+    for (int i = 0; i < viewsToSort.size(); i++) {
+      viewsToSort.get(i).bringToFront();
+    }
+    view.invalidate();
   }
 
   public int getChildCount(T parent) {

ReactAndroid/src/main/java/com/facebook/react/views/view/ReactViewManager.java

@@ -195,6 +195,7 @@ public void addView(ReactViewGroup parent, View child, int index) {
     } else {
       parent.addView(child, index);
     }
+    reorderChildrenByZIndex(parent);
   }
 
   @Override