Delay the PiP menu attachment

The flash at the end of entering PiP transition in gesture navigation
mode is a regression from ag/12997165. Fixed by delaying
PipMenuController#attach late (when the leash is set to transparent) in
the process to avoid any artifacts on the leash caused by addShellRoot.

Video: http://rcll/aaaaaabFQoRHlzixHdtY/dFdpJHvTCP6NmNEkDHrtei
Bug: 174814759
Test: see video
Change-Id: I7e74253f94b16977d04e27c2ab76fd3178001630