Workaround to ensure that a SystemUI process is always available.

- For a non-primary user, this CL will ensure that the SystemUI process
  is started when we are switched to the user.  This allows us to
  maintain our current user-management model for Recents, which depends
  on this process for preloading and state management.

Bug: 27175589
Change-Id: Id985fc2876e6daf06f303b44c0f9d1d3fd377842