Same here.
I still get this error in CrashLytics on an Android 8.1.0 device (Nexus 5, device state: background) after adding the two #additionaljar lines listed above.
Fatal Exception: java.lang.RuntimeException: Unable to instantiate receiver com.google.firebase.iid.FirebaseInstanceIdReceiver: java.lang.ClassNotFoundException: Didn't find class "com.google.firebase.iid.FirebaseInstanceIdReceiver" on path: DexPathList[[zip file "/data/app/xxx/base.apk"],nativeLibraryDirectories=[/data/app/xxx/lib/x86, /system/lib, /vendor/lib]]
at android.app.ActivityThread.handleReceiver(ActivityThread.java:3171)
at android.app.ActivityThread.-wrap17()
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1672)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
Caused by java.lang.ClassNotFoundException: Didn't find class "com.google.firebase.iid.FirebaseInstanceIdReceiver" on path: DexPathList[[zip file "/data/app/xxx/base.apk"],nativeLibraryDirectories=[/data/app/xxx/lib/x86, /system/lib, /vendor/lib]]
at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:125)
at java.lang.ClassLoader.loadClass(ClassLoader.java:379)
at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
at android.app.ActivityThread.handleReceiver(ActivityThread.java:3166)
at android.app.ActivityThread.-wrap17()
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1672)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
Thanks,
Johannes