Workaround issue with pip overlay not hiding cleanly.

- We were just finishing the activity which was causing issues with the
  wrong app transition being applied.  We actually can just hide the menu
  activity first before finishing it like we do elsewhere.

Bug: 38222481
Test: Open PiP, launch to show assistant

Change-Id: I348f8a17f76006bcef46fe01c87deae0b89f6586