Fix issue with PiP callbacks not being handled

For secondary users, stopping primary user process
is not necessary anymore since PiP is managed
in the primary user's SystemUI process,
this logic was prevening the relaunching of the activity.

Test: Launch PiP from a secondary user on TV device,
      restart the activity and ensure that it is brought
      to fullscreen.

Change-Id: I3a0658c9f55f85771b92a71a1ddeb0f9d544e869