Bump up preloading recents on boot up to user unlocked.

- To fix b/29320695, we moved the initial recents preloading to boot 
  completed.  However, that event happens fairly late in the boot 
  sequence, and preloading can instead be done in response to user 
  unlocked.

Bug: 29879478
Change-Id: I881bd9c4c3b91d73df4947bfd2cc55eb6e615d15