Fix issue #7295951: Mako asks which Launcher to use at every re-boot

A couple problems:

- We need to clear app preferences later, now that we have encrypted apps.
- The multi-user implementation of this would allow different preferred
  apps from different users to potentially interefere with each other.
  They are not completely separate data structures.

Change-Id: Id4f1ebb6414fdf30ff1049adaa1efe83dabac01a