Don't let PIP window control system bars

Fix: 267413713
Test: 1. Watch a YouTube video on Chrome.
      2. Enter fullscreen mode.
      3. Swipe to show navigation bar and go to home screen.
      4. Open Calculator while PIP is playing the video.
      See if system bars are transiently hidden while opening
      Calculator.
Change-Id: I918f35435d125c80d986d6f1c715f0129142edae