Enter PIP when exiting prev PIP with Shell transition

Before, the dismissPip() is not collected in the enter PIP transition.
Now, the Task that leaving PIP windowing mode will also be part of the
TRANSIT_PIP transition.

Fix: 213300721
Test: manual test with two Pip apps
Change-Id: Iaab34a7969ff4cb5d9768754d643d5f32913f57a