Get the non-primary pointer id that just went down (returns -1 or > 0)

Property definitions

android $ NativeAndroidMotionEvent :: index_down_pointer
	# Get the non-primary pointer id that just went down (returns -1 or > 0)
	fun index_down_pointer: Int `{
		int a = AMotionEvent_getAction(self);
		if ((a & AMOTION_EVENT_ACTION_MASK) == AMOTION_EVENT_ACTION_POINTER_DOWN)
			return (a & AMOTION_EVENT_ACTION_POINTER_INDEX_MASK) >> AMOTION_EVENT_ACTION_POINTER_INDEX_SHIFT;
		else return -1;
	`}
lib/android/input_events.nit:64,2--70,3