Save a copy of DisplayMetrics native map in DeviceInfoModule

Summary:
After some more testing, I discovered a problem in D19395326 because the native map that DeviceInfoModule was storing in `mPreviousDisplayMetrics` had been consumed when the event was emitted to JS. This caused the comparison to fail, so it would emit the event again when the dimensions hadn't changed.

In this diff, I'm storing a Java-only copy of the native map before emitting the event to JS so this shouldn't happen.

Changelog: [Android][Fixed] Fix bug in updating dimensions in JS

Reviewed By: mdvacca

Differential Revision: D19462861

fbshipit-source-id: 2e47479df93377b85fe87f255972dd31e874e3a8

Author: Emily Janzer

ReactAndroid/src/main/java/com/facebook/react/modules/deviceinfo/DeviceInfoModule.java

@@ -90,10 +90,10 @@ public void emitUpdateDimensionsEvent() {
       WritableNativeMap displayMetrics =
           DisplayMetricsHolder.getDisplayMetricsNativeMap(mFontScale);
       if (!displayMetrics.equals(mPreviousDisplayMetrics)) {
+        mPreviousDisplayMetrics = displayMetrics.copy();
         mReactApplicationContext
             .getJSModule(DeviceEventManagerModule.RCTDeviceEventEmitter.class)
             .emit("didUpdateDimensions", displayMetrics);
-        mPreviousDisplayMetrics = displayMetrics;
       }
     } else {
       ReactSoftException.logSoftException(