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
         }
     }
 }

android/app/src/main/AndroidManifest.xml

@@ -1,6 +1,9 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="de.johrpan.musicus">
 
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
+    <uses-permission android:name="android.permission.WAKE_LOCK" />
+
     <application
         android:name="io.flutter.app.FlutterApplication"
         android:label="Musicus"
@@ -21,6 +24,18 @@
             </intent-filter>
         </activity>
 
+        <service android:name="com.ryanheise.audioservice.AudioService">
+            <intent-filter>
+               <action android:name="android.media.browse.MediaBrowserService" />
+            </intent-filter>
+        </service>
+
+        <receiver android:name="androidx.media.session.MediaButtonReceiver" >
+            <intent-filter>
+               <action android:name="android.intent.action.MEDIA_BUTTON" />
+            </intent-filter>
+        </receiver>
+
         <meta-data
             android:name="flutterEmbedding"
             android:value="2" />