title: Build fails when AppsFlyer is used with ProGuard
body: <p>I'm following <a href="https://support.appsflyer.com/hc/en-us/articles/207032126-AppsFlyer-SDK-Integration-Android" rel="nofollow noreferrer">this manual</a> to integrate AppsFlyer into our Android app.</p>
  
  <p>When I try to build the project with <code>minifyEnabled true</code> the following warnings appear in the build log:</p>
  
  <blockquote>
    <p>Warning: com.appsflyer.GcmInstanceIdListener: can't find superclass or interface com.google.android.gms.iid.InstanceIDListenerService</p>
    
    <p>Warning: com.appsflyer.GcmInstanceIdListener: can't find referenced class com.google.android.gms.iid.InstanceID</p>
    
    <p>Warning: com.appsflyer.GcmInstanceIdListener: can't find referenced class com.google.android.gms.iid.InstanceIDListenerService
    Warning: com.appsflyer.GcmInstanceIdListener: can't find referenced class com.google.android.gms.iid.InstanceID
    Warning: com.appsflyer.GcmInstanceIdListener: can't find referenced class com.google.android.gms.iid.InstanceID</p>
    
    <p>Warning: com.appsflyer.GcmInstanceIdListener: can't find referenced class com.google.android.gms.iid.InstanceIDListenerService</p>
    
    <p>Warning: com.appsflyer.GcmInstanceIdListener: can't find referenced class com.google.android.gms.iid.InstanceIDListenerService</p>
    
    <p>Warning: com.appsflyer.u: can't find referenced class com.google.android.gms.iid.InstanceIDListenerService</p>
  </blockquote>
  
  <p>And the build fails.</p>
  
  <p>Here is relevant part of the app's module build.gradle:</p>
  
  <pre><code>dependencies {
      implementation 'com.google.firebase:firebase-core:16.0.7'
      implementation 'com.google.firebase:firebase-messaging:17.3.4'
      implementation 'com.appsflyer:af-android-sdk:4+@aar'
      implementation 'com.android.installreferrer:installreferrer:1.0'
  }
  </code></pre>
  
  <p>Here is relevant part of ProGuard configuration:</p>
  
  <pre><code>-dontwarn com.android.installreferrer
  </code></pre>