Add missing using directive for AsNonNull in OsuGameActivity.cs

The previous commit introduced the use of `.AsNonNull()` but lacked
the necessary `using osu.Framework.Extensions.ObjectExtensions;`
directive, causing a CI failure.

Author: google-labs-jules[bot]

osu.Android/OsuGameActivity.cs

@@ -7,6 +7,7 @@
 using System.Reflection;
 using System.Threading.Tasks;
 using osu.Framework.Android;
+using osu.Framework.Extensions.ObjectExtensions;
 using osu.Framework.Extensions;
 using osu.Game.Database;
 using Debug = System.Diagnostics.Debug;