[Offline indicator v2] Fix NullPointerException

After switching themes (dark/light), the ConnectivityDetector instance
would not be destroyed since it's added as an observer to the
NetworkChangeNotifier singleton. This was keeping a lot of objects
around and causing invalid status indicator instances to access a null
activity reference through a lambda created in TabbedRootUiCoordinator.

This CL adds a #destroy method to ConnectivityDetector and a call to
it from OfflineIndicatorControllerV2#destroy().

Bug: 1070386
Change-Id: Iee7a9cb913fa8aa62960f602e6f45242e0cc60e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2150026Reviewed-by: Jian Li <jianli@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Sinan Sahin <sinansahin@google.com>
Cr-Commit-Position: refs/heads/master@{#759357}

chrome/android/java/src/org/chromium/chrome/browser/offlinepages/indicator/ConnectivityDetector.java

@@ -228,6 +228,10 @@ public class ConnectivityDetector implements NetworkChangeNotifier.ConnectionTyp
         detect();
     }
 
+    public void destroy() {
+        NetworkChangeNotifier.removeConnectionTypeObserver(this);
+    }
+
     public void detect() {
         onConnectionTypeChanged(NetworkChangeNotifier.getInstance().getCurrentConnectionType());
     }

chrome/android/java/src/org/chromium/chrome/browser/offlinepages/indicator/OfflineIndicatorControllerV2.java

@@ -73,6 +73,9 @@ public class OfflineIndicatorControllerV2 implements ConnectivityDetector.Observ
     }
 
     public void destroy() {
-        mConnectivityDetector = null;
+        if (mConnectivityDetector != null) {
+            mConnectivityDetector.destroy();
+            mConnectivityDetector = null;
+        }
     }
 }