Add an invisible layer to trap focus

This is a work around for b/169372603 where PiP menu defaults the focus
on expand button when being attached to system window.

There is another inconsistency observed regarding of touch feedback
which may not be related to b/169372603 but worth mentioning
- when entering PiP and close button is shown, tap on PiP window shows
  visual touch feedback
- when PiP menu in closed state, eg. nothing is shown in menu, tap on
  PiP window shows no visual touch feedback

Video: http://rcll/aaaaaabFQoRHlzixHdtY/cLwTz7ad7P9nrrqk1uPiRP
Bug: 169372603
Test: Enter PiP from YT, see video
Change-Id: Ief8976fac28a65ac35724adcad7f25e11f1876a5