Add enable flag for Android device orientation change event.

There are reports (not reproducible with appRtcDemo) that
outstanding device orientation change event
OrientationEventListener.onOrientationChanged can be
triggered even after these events are disabled by
OrientationEventListener.disable() code.
Avoid calling native code in this case since underlying
C++ class may have already been deleted.

BUG=3564
R=braveyao@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23609004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7172 4adac7df-926f-26a2-2b94-8c16560cd09d