Use elevation to implement shadows on Android

Summary: This PR includes a working interface to Android's `elevation` property implemented as an RN style. Elevation is the only (easy) [platform-supported way to create shadows](http://developer.android.com/training/material/shadows-clipping.html). For it to work note that you must be running on Android 5.0+, and add `elevation` to a view with a `backgroundColor` set. These are platform limitations.

This PR is not intended to be merged in its current state, but rather to inform the discussion from #2768. At a minimum, before merging we would need to add the elevation style to the docs and rebase this to master (**EDIT** I have now rebased on master because from v0.14.2 too many commits were being pulled in -- haven't tested it since the rebase though). Additionally, it might be good to add tests, although I couldn't find any for the Android code. I'm happy to get that done if this feature gets signed off by the React Native team.

Finally, as I argued in #2768 I think that `elevation` is a useful abstraction ov
Closes https://github.com/facebook/react-native/pull/4180

Reviewed By: svcscm

Differential Revision: D2684746

Pulled By: mkonicek

fb-gh-sync-id: 825f3ccd20c4b0eea9d11b5f0e3a6b018b7e4378

Libraries/Components/View/ViewStylePropTypes.js

@@ -47,6 +47,14 @@ var ViewStylePropTypes = {
   ),
   shadowOpacity: ReactPropTypes.number,
   shadowRadius: ReactPropTypes.number,
+  /**
+   * (Android-only) Sets the elevation of a view, using Android's underlying 
+   * [elevation API](https://developer.android.com/training/material/shadows-clipping.html#Elevation).
+   * This adds a drop shadow to the item and affects z-order for overlapping views.
+   * Only supported on Android 5.0+, has no effect on earlier versions.
+   * @platform android
+   */
+  elevation: ReactPropTypes.number,
 };
 
 module.exports = ViewStylePropTypes;

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

@@ -17,12 +17,14 @@ import android.graphics.Canvas;
 import android.graphics.Color;
 import android.graphics.ColorFilter;
 import android.graphics.DashPathEffect;
+import android.graphics.Outline;
 import android.graphics.Paint;
 import android.graphics.Path;
 import android.graphics.PathEffect;
 import android.graphics.Rect;
 import android.graphics.RectF;
 import android.graphics.drawable.Drawable;
+import android.os.Build;
 
 import com.facebook.react.common.annotations.VisibleForTesting;
 import com.facebook.csslayout.CSSConstants;
@@ -123,6 +125,23 @@ import com.facebook.csslayout.Spacing;
     return ColorUtil.getOpacityFromColor(ColorUtil.multiplyColorAlpha(mColor, mAlpha));
   }
 
+  /* Android's elevation implementation requires this to be implemented to know where to draw the shadow. */
+  @Override
+  public void getOutline(Outline outline) {
+    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
+      super.getOutline(outline);
+      return;
+    }
+    if(!CSSConstants.isUndefined(mBorderRadius) && mBorderRadius > 0) {
+      float extraRadiusFromBorderWidth = (mBorderWidth != null)
+              ? mBorderWidth.get(Spacing.ALL) / 2f
+              : 0;
+      outline.setRoundRect(getBounds(), mBorderRadius + extraRadiusFromBorderWidth);
+    } else {
+      super.getOutline(outline);
+    }
+  }
+
   public void setBorderWidth(int position, float width) {
     if (mBorderWidth == null) {
       mBorderWidth = new Spacing();

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

@@ -64,6 +64,14 @@ public class ReactViewManager extends ViewGroupManager<ReactViewGroup> {
     view.setBorderStyle(borderStyle);
   }
 
+  @ReactProp(name = "elevation")
+  public void setElevation(ReactViewGroup view, float elevation) {
+    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
+      view.setElevation(PixelUtil.toPixelFromDIP(elevation));
+    }
+    // Do nothing on API < 21
+  }
+
   @ReactProp(name = "pointerEvents")
   public void setPointerEvents(ReactViewGroup view, @Nullable String pointerEventsStr) {
     if (pointerEventsStr != null) {