Remove FLAG_ACTIVITY_NEW_TASK when startActivityAsUser

In the previous CL, we only remove the flag when the transition is
enabled. This CL remove it entirely regardless of the existing of the
transition since we'll be using the default transition.

Bug: 186858442
Test: click on any work profile app and don't see task transition.
Change-Id: Ie93e87156ca49e4bccef28d75afc54161919da62