test-app: opt-in unstable API usage to fix lint

Apparently setDurationMs is marked as unstable and used in the onConfigurationChanged callback in MainActivity

Author: crazo7924

test-app/src/main/java/net/newpipe/newplayer/testapp/MainActivity.kt

@@ -202,6 +202,7 @@ class MainActivity : AppCompatActivity() {
      * In a pure compose environment such reconfigurations are not necessary. Since NewPipe
      * might not be fully Compose yet, such hacks are required.
      */
+    @OptIn(UnstableApi::class)
     override fun onConfigurationChanged(newConfig: Configuration) {
         super.onConfigurationChanged(newConfig)
         if (currentOrientation != newConfig.orientation) {