[Trimming] Remove RegisterJniNatives suppression

[simonrozsival]

Summary

• remove the local UnconditionalSuppressMessage around Type.GetType() in RegisterJniNatives()
• mark the native callback with RequiresUnreferencedCode instead of hiding the reflection warning

Part of #10794.

Validation

• ./dotnet-local.sh build src/Mono.Android/Mono.Android.csproj -v:minimal -nr:false succeeded
• no IL/trimming warnings (IL####) found in the build log

Note: this fresh worktree emitted existing generated/baseline warnings unrelated to this one-file change (CS0436/CA1422/RS0025/javac warnings).

[Copilot]

Pull request overview

This PR updates JNIEnvInit.RegisterJniNatives() to stop suppressing trimming diagnostics around Type.GetType() and instead explicitly annotates the unmanaged callback as trimming-unsafe via RequiresUnreferencedCode, aligning with the goal of removing UnconditionalSuppressMessage usage.

Changes:

• Removed the local UnconditionalSuppressMessage + DynamicallyAccessedMembers-annotated wrapper around Type.GetType().
• Added [RequiresUnreferencedCode] to the RegisterJniNatives unmanaged callback to surface the trimming hazard rather than suppress it.

[simonrozsival]

Note: We might want to set IsAotCompatible to false when the type map is not the trimmable type map.

[simonrozsival]

This method is not used with the trimmable typemap and so it will be trimmed out when it's not used with the trimmable type map. I'm closing this now and I won't be looking into this area at this moment.