Fix pip activity remained in PAUSING state

The enter-pip activity was reparented to a newly created task
because the original host task contains more than one activity.
However, the pip activity remained in PAUSING state because
the task pausing activity was null and therefore the activity
didn't schedule to pause.

Bug: 213541422
Test: atest PinnedStackTests
Change-Id: I1aea45f70eb3983dc9859585321258813ee62af7