Fixed bug in detection of "natural" orientation of Android device.

Value of "natural" orientation is computed on first OnOrientationChanged() call (not in ctor, because it may return invalid value if called too early).
Orientation listener is active between Activity.OnResume()/OnPause() calls.
Orientation value of OrientationUnknown (-1) is ignored.
Merge pull request #2658 from Andreyul/orientation

Fixed bug in detection of "natural" orientation of Android device.