Added a new method to Context: startActivityAsUser() requiring the
INTERACT_ACROSS_USERS_FULL permission.
Show the correct Recents list, based on current user.
Added a getRecentTasksForUser() in ActivityManager. Hidden and requires
the INTERACT_ACROSS_USERS_FULL permission.
Change-Id: If5b56465efdd3ead36601a3b51ed4af157bbf35c