title: Firebase notifications not working in Android Emulator
body: <p>I am developing an Android app which uses Firebase to send notifications. Everything works as expected when using my physical device (Google Nexus 5, and also tested on others), but I cannot receive notification when using the emulator.</p>
  
  <p>Platform: Windows
  Android Studio: 2.3.1 with Google Play Services 39
  API: 24
  Emulator: Nexus 6 API 24, Android 7.0</p>
  
  <p>Any idea?</p>