Fix Android startup crash caused by JNI activation failure in trimmed builds

[google-labs-jules]

Aggressive .NET trimming in .NET 10 was stripping the JNI activation constructor for OsuGameActivity, leading to a System.NotSupportedException on launch.

Changes:

• Added explicit JNI constructor (IntPtr, JniHandleOwnership) to OsuGameActivity.
• Migrated game and input handler initialization to the JNI constructor.
• Added [UnconditionalSuppressMessage] to OsuGameActivity to silence trimmer warnings.
• Updated Linker.xml to preserve all types in the osu.Android assembly.
• Added Linker preservation for both possible AndroidGameActivity namespaces to ensure compatibility with framework versions.

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PR created automatically by Jules for task 17469659687026419914 started by @winnerspiros

[google-labs-jules]

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