Remove unecessary code from ActivityStack

   - Due to a previous CL to allow activities to be visible under
   the Home stack, some code un ActivityStack became unecessary.
   - This CL removes the uused code and changes the interface of
   ensureActivitiesVisibleLocked, since we no longer need to
   special case visibility for the Home stack.

Change-Id: I5cffe903bc82041fe79bfc63107df71ddf01232c