Fix NPE in onVisibilityChanged

 Bug: 15089790
 onVisibilityChanged may happen before the View is attached.
 Only attempt to re-set the update listener if mLayer is not null,
 as it means the View is attached and has already drawn once. If
 mLayer is null then the update listener will be set when the view
 next draws

Change-Id: I406e359a0a0720988f026f9cbde26afdb564ca92