Prevent potential NPE when updating state to bar manager

If the process of bar window is died, the associated window
will be set to null. Since BarController is associated with
display, the display id can be a final field, so it no longer
depends on the nullable member mWin.

Fixes: 129240719
Test: manual - Play video in landscape mode and make systemui
      crash, e.g. adb shell kill -11 ${pid of systemui}. The
      result should be only the systemui is restarted.

Change-Id: If5b93e76cb856dd0dd80fb8523788f4587222c9c