Notification from Android, the activity is being destroyed

Clean up and exit.

Property definitions

android $ Activity :: on_destroy
	# Notification from Android, the activity is being destroyed
	#
	# Clean up and exit.
	fun on_destroy
	do
		app.on_destroy

		native.delete_global_ref
		app.activities.remove self
	end
lib/android/nit_activity.nit:266,2--275,4