fix(admob): AdEventHandler returns javascript (not native) unsubscrib…

…e function (#4920)

* [bug] Admob listener wrong unsibscribe function

`_setAdEventHandler` method return a wrong unsubscribe function. It should remove js listener, but instead it unsubscribe ad instance from native events and _onAdEventHandler is still could be used by the instance

* fix(AdMob): fix linter error

packages/admob/lib/ads/MobileAd.js

@@ -61,7 +61,7 @@ export default class MobileAd {
 
   _setAdEventHandler(handler) {
     this._onAdEventHandler = handler;
-    return () => this._nativeListener.remove();
+    return () => (this._onAdEventHandler = null);
   }
 
   get adUnitId() {