Fixes orientation changes for Android Tablets.
Fix for orientation when Activity closed.

Android Activity still lives, so the OnOrientationChanged event is still
executed, even thou AndroidGameActivity.Game is null. This commit fixes
this issue. Since it's a very small fix and related to the same method
of the first commit, I just added another commit to the same PR.
Merge pull request #1987 from azchohfi/androidOrientationFixForTablets

Fixes orientation changes for Android Tablets.