Update the cached dimensions when orientation changes (facebook#30324)

Summary:
Currently the dimensions are created once, and then cached.  This change will reload the dimensions when the device orientation changes to insure that dimension update events follow orientation changed events.

this should help address the following issues, that I know of:
facebook#29105
facebook#29451
facebook#29323

## Changelog

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[Android] [Fixed] - Dimension update events are now properly sent following orientation change

Pull Request resolved: facebook#30324

Test Plan: Open up RNTester app.  Select the Dimensions API list item.  Rotate the device and verify that the dimensions are correct based on orientation.

Reviewed By: fkgozali

Differential Revision: D24874733

Pulled By: ejanzer

fbshipit-source-id: 867681ecb009d368a2ae7b67d94d6355e67dea7b

ReactAndroid/src/main/java/com/facebook/react/ReactRootView.java

@@ -756,6 +756,7 @@ private void checkForDeviceOrientationChanges() {
         return;
       }
       mDeviceRotation = rotation;
+      DisplayMetricsHolder.initDisplayMetrics(getContext().getApplicationContext());
       emitOrientationChanged(rotation);
     }