Allowing system uid to embed Task on secondary user

Settings app was unable to embed task on secondary user
because the uid of Settings was composed with user_id
and app_id, which was not 1000.

Allowing task embedding for system uid on all users.
Also specify the user id while starting activity in
TaskFragments.

Bug: 201738029
Test: start Settings on secondary user
Change-Id: I404379ed049f15abf53084d381c169d3d4cb1166