Cancel callbacks for messaging (#745)

* Cancel callbacks for messaging

util::Terminate is referenced counted som when there ar more APIs than messaging active the callbacks will not be canceled until later and still cause a NULL ref due to the FutureData being destroyed now.

* Cancel callback earlier

* Update readme

Author: TBarendt

messaging/src/android/cpp/messaging.cc

@@ -677,6 +677,7 @@ void Terminate() {
   }
   internal::UnregisterTerminateOnDefaultAppDestroy();
   JNIEnv* env = g_app->GetJNIEnv();
+  util::CancelCallbacks(env, kApiIdentifier);
   // Dereference the app.
   {
     MutexLock lock(g_app_mutex);

release_build_files/readme.md

@@ -577,6 +577,8 @@ code.
         ([#737](https://github.com/firebase/firebase-cpp-sdk/issues/737))
     -   Messaging (Android): Fixed crash during termination.
         ([#739](https://github.com/firebase/firebase-cpp-sdk/pull/739))
+        ([#745](https://github.com/firebase/firebase-cpp-sdk/pull/745))
+
 
 ### 8.7.0
 -   Changes