Fixes orientation changes for Android Tablets.

This fixes the orientation changed event for Android Tablet.
This solution was found on MonoGame's CodePlex discussions.
I only adapted the solution to work on the actual MonoGame's version. All the credit goes to JohnHolmes.
I found the bug and, coincidentally, found the solution on the discussion board.
The error report:
https://monogame.codeplex.com/discussions/452578
And the fix, by JohnHolmes:
https://monogame.codeplex.com/discussions/444056
I just packet it all together, made a PR and submitted.
I've also tested this on a Samsung Galaxy Tab 2. Works flawlessly.