Use new DisplayMetrics object in getDisplayMetrics as to not overwrite Android global state

Summary:
The change in facebook@8e60394 actually causes a super bad bug when `context.getResources().getDisplayMetrics()` is accessed in other parts of the application.

It turns out that when you dive into the impl of `getRealMetrics`, it mutates whatever `DisplayMetrics` object is passed to it. In this case, `getDisplayMetrics` ends up mutating the `DisplayMetrics` object that sits on the application context's `Resources` instance.

This PR makes that not so.

/cc jesseruder ide jaysoo bestander astreet
Closes facebook#5764

Reviewed By: svcscm

Differential Revision: D2902386

Pulled By: androidtrunkagent

fb-gh-sync-id: 3f24b68bc7e6b4ca83808c03ef3637e1ac9a673e

Author: skevy

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

@@ -464,7 +464,8 @@ public void sendAccessibilityEvent(int tag, int eventType) {
   private DisplayMetrics getDisplayMetrics() {
     Context context = getReactApplicationContext();
 
-    DisplayMetrics displayMetrics = context.getResources().getDisplayMetrics();
+    DisplayMetrics displayMetrics = new DisplayMetrics();
+    displayMetrics.setTo(context.getResources().getDisplayMetrics());
     WindowManager wm = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
     Display display = wm.getDefaultDisplay();