Fix DisplayCutoutTest

The test was removing an observer after finishing the Activity, leading
to an NPE.

Bug: 1146208
Change-Id: I59563b4d5c9c9501d1f3a3a64c1014f39fa0351f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522607
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Auto-Submit: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824948}

chrome/android/javatests/src/org/chromium/chrome/browser/display_cutout/DisplayCutoutTestRule.java

@@ -176,7 +176,9 @@ public class DisplayCutoutTestRule<T extends ChromeActivity> extends ChromeActiv
     }
 
     protected void tearDown() {
-        getActivity().getFullscreenManager().removeObserver(mListener);
+        if (!getActivity().isActivityFinishingOrDestroyed()) {
+            getActivity().getFullscreenManager().removeObserver(mListener);
+        }
         mTestServer.stopAndDestroyServer();
     }