Called when you are no longer visible to the user. You will next receive either Activity.OnRestart, Activity.OnDestroy, or nothing, depending on later user activity.
Note that this method may never be called, in low memory situations where the system does not have enough memory to keep your activity's process running after its Activity.OnPause method is called.
Derived classes must call through to the super class's implementation of this method. If they do not, an exception will be thrown.