Attempt to fix NPE when reparent to the original task.

7f64b144, Fix back stack for PiP from multi-activity Task. Trying to
reparent pinned activity back to its original task. The NPE is
introduced when the original task is removed.

This CL removes the pinned task when the original task is to be
removed and checks for the task is attached before doing the
re-parent.

Bug: 186374793
Test: atest PinnedStackTests
Test: atest RootWindowContainerTests
Change-Id: I6ea6100cb94b3982737c89f68de824a14415283a