Handle GCed WeakReferences in removeObserver

WeakReferences always need to be null checked when getting it's value as
it may have been garbage collected.

Keeping the fix simple here for a fast fix. Kotlin has built-in
observables so any future investment in this class should be done
migrating to this.

Author: jkasten2

OneSignalSDK/onesignal/src/main/java/com/onesignal/OSObservable.java

@@ -55,6 +55,7 @@ void addObserverStrong(ObserverType observer){
    void removeObserver(ObserverType observer) {
       for(int i = 0; i < observers.size(); i++) {
          Object anObserver = ((WeakReference)observers.get(i)).get();
+         if (anObserver == null) continue;
          if (anObserver.equals(observer)) {
             observers.remove(i);
             break;