musicus_player: Add basic playback functionality

musicus_player/android/src/main/kotlin/de/johrpan/musicus_player/MusicusPlayerPlugin.kt

@@ -1,5 +1,9 @@
 package de.johrpan.musicus_player
 
+import android.content.Context
+import android.media.MediaPlayer
+import android.net.Uri
+
 import androidx.annotation.NonNull;
 
 import io.flutter.embedding.engine.plugins.FlutterPlugin
@@ -11,14 +15,41 @@ import io.flutter.plugin.common.PluginRegistry.Registrar
 
 public class MusicusPlayerPlugin: FlutterPlugin, MethodCallHandler {
   private lateinit var channel: MethodChannel
+  private lateinit var context: Context
+
+  private var uri: Uri? = null
+  private var mediaPlayer: MediaPlayer? = null
 
   override fun onAttachedToEngine(@NonNull flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
     channel = MethodChannel(flutterPluginBinding.getFlutterEngine().getDartExecutor(), "de.johrpan.musicus_player/platform")
-    channel.setMethodCallHandler(this);
+    channel.setMethodCallHandler(this)
+    context = flutterPluginBinding.getApplicationContext()
   }
 
   override fun onMethodCall(@NonNull call: MethodCall, @NonNull result: Result) {
-    // TODO: Implement.
+    if (call.method == "setUri") {
+      val newUri = Uri.parse(call.argument<String>("uri"))
+      uri = newUri
+
+      if (mediaPlayer != null) {
+        mediaPlayer?.release()
+      }
+
+      mediaPlayer = MediaPlayer.create(context, uri)
+      mediaPlayer?.setOnCompletionListener {
+        channel.invokeMethod("onComplete", null)
+      }
+
+      result.success(null)
+    } else if (call.method == "play") {
+      mediaPlayer?.start()
+      result.success(null)
+    } else if (call.method == "pause") {
+      mediaPlayer?.pause()
+      result.success(null)
+    } else {
+      result.notImplemented()
+    }
   }
 
   override fun onDetachedFromEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {