Keyguard: Ensure that a widget and page is attached before attaching the ChallengeLayout.OnBouncerStateChangedListener

otherwise it can result in a NPE due that the page was not loaded. Especially on low end
devices this can happen (see zoomOutToBouncer() method @KeyguardWidgetPager)

Change-Id: I7e940eb2fdf885f158f4c5ff2872ebafcda76da0