Fix Xamarin.Jetbrains.Annotations conflicting with Jetbrains.Annotations

[Susko3]

This conflict was most noticeable when using [NotNull] in osu! ppy/osu@9c22fa3.

This doesn't fix the conflict in osu.Framework.Android, but it does fix it for downstream projects. If a fix is needed for o!f, PackageReference Aliases can be used.

I've tested osu! with a local framework checkout and with a local package, and it works fine without the workaround.

---

• Upstream issue (wontfix): Xamarin.Jetbrains.Annotations namespace clash with JetBrains.Annotations causes CS0433 build failure xamarin/AndroidX#885

The namespace conflict can be tested with this diff (compile error on master, works fine with this PR):

diff --git a/osu.Framework.Tests.Android/TestGameActivity.cs b/osu.Framework.Tests.Android/TestGameActivity.cs
index 19851817f..8dfae3995 100644
--- a/osu.Framework.Tests.Android/TestGameActivity.cs
+++ b/osu.Framework.Tests.Android/TestGameActivity.cs
@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using Android.App;
+using JetBrains.Annotations;
 using osu.Framework.Android;
 
 namespace osu.Framework.Tests.Android
@@ -9,6 +10,7 @@ namespace osu.Framework.Tests.Android
     [Activity(ConfigurationChanges = DEFAULT_CONFIG_CHANGES, Exported = true, LaunchMode = DEFAULT_LAUNCH_MODE, MainLauncher = true)]
     public class TestGameActivity : AndroidGameActivity
     {
+        [NotNull]
         protected override Game CreateGame()
             => new VisualTestGame();
     }