Fix overlay removal bug (react-native-maps#2197)

Author: kabbi

lib/android/src/main/java/com/airbnb/android/react/maps/AirMapOverlay.java

@@ -100,6 +100,7 @@ public void addToMap(GoogleMap map) {
 
   @Override
   public void removeFromMap(GoogleMap map) {
+    this.map = null;
     if (this.groundOverlay != null) {
       this.groundOverlay.remove();
       this.groundOverlay = null;