Deferring the move to the default screen until after we callback to the callbacks.

- Since move to default screen starts the scroller immediately, any delays in
  handling onNewIntent and onHomeIntent in the callbacks will cause the scroller
  to skip frames the next time it updates.  This change will defer updating the
  page to the default screen until after onNewIntent is called back (and all its
  posted runnables).

Bug: 22929080
Change-Id: Ibab6106938721702d4da23faaca99039861e10dc