Add player skeleton and playback service

This introduces a dependency on audio_service and implements the
playback service using that package. The UI was adapted to the new
interface.

android/app/build.gradle

@@ -50,6 +50,8 @@ android {
             // TODO: Add your own signing config for the release build.
             // Signing with the debug keys for now, so `flutter run --release` works.
             signingConfig signingConfigs.debug
+            // See https://github.com/ryanheise/audio_service/blob/master/README.md#android-setup
+            shrinkResources false
         }
     }
 }