Classify PINNED as a transient windowing mode.

Since we now perform setWindowingMode for PIP (even for the case of
re-parenting, we move the activities to a new stack first and then set
its windowing mode to PINNED), we can now add this condition.

Bug: 114842032
Bug: 144508210
Test: Try to do repro steps on b/144508210, no longer happens.
Change-Id: I0e491c52273e9d5edef05b1745c5fbffd9361ac0