Prevent invisible PiP menu activity from having touch focus.

- Mark the decor view as invisible when the the PiP menu activity is
  hidden.  We don't call Activity.setVisible() as that alters other
  internal states that affect the system's understanding of whether
  the activity will draw or not.

Bug: 36225602
Test: android.server.cts.ActivityManagerPinnedStackTests

Change-Id: I6b1ed7098e1f0ff427f0655c45e566ca60979623