"Error Implementing AudioServiceActivity in Flutter: Unresolved Package 'ryanheise'"

Question

I'm trying to implement audio_services in my app to read podcast while device is locked,

When I try to change this :

 <!-- EDIT THE android:name ATTRIBUTE IN YOUR EXISTING "ACTIVITY" ELEMENT -->
    <activity android:name="com.ryanheise.audioservice.AudioServiceActivity" ...>

I get this error :

Class referenced in the manifest, com.ryanheise.audioservice.AudioServiceActivity, was not found in the project or the libraries
Unresolved package 'ryanheise' 

I've tried numerous solutions, but nothing seems to fix this issue. Here is the relevant portion of my pubspec.yaml:

environment:
  sdk: '>=2.19.6 <3.0.0'
...
dependencies:
  flutter:
    sdk: flutter
  record: ^4.4.4
  audio_service: ^0.18.10
  ffmpeg_kit_flutter: ^5.1.0

I Tried

flutter clean and flutter build
flutter flutter pub cache repair
...

restarting Android Studio ...

What could be causing this error, and how can I fix it? Any help would be greatly appreciated.