Add ReadOnlySpan<byte> overloads for JNI member lookups

[simonrozsival]

Summary

Add ReadOnlySpan<byte> overloads for JNI class, method, and field lookups. Callers can pass UTF-8 string literals (e.g., "java/lang/Object"u8) to skip Marshal.StringToCoTaskMemUTF8 / ZeroFreeCoTaskMemUTF8 entirely, eliminating the per-call UTF-16→UTF-8 transcoding and native heap alloc/free.

Contributes to dotnet/android#11885

New API surface

All overloads are additive (existing string APIs unchanged) and only compiled under FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS (the .NET Android runtime path).

JniEnvironment (low-level):

• Types.FindClass(ReadOnlySpan<byte>) / TryFindClass(ReadOnlySpan<byte>, out JniObjectReference)
• InstanceMethods.GetMethodID(JniObjectReference, ReadOnlySpan<byte>, ReadOnlySpan<byte>)
• StaticMethods.GetStaticMethodID(JniObjectReference, ReadOnlySpan<byte>, ReadOnlySpan<byte>)
• InstanceFields.GetFieldID(JniObjectReference, ReadOnlySpan<byte>, ReadOnlySpan<byte>)
• StaticFields.GetStaticFieldID(JniObjectReference, ReadOnlySpan<byte>, ReadOnlySpan<byte>)

JniType (high-level):

• Constructor: JniType(ReadOnlySpan<byte>)
• GetConstructor, GetInstanceMethod, GetStaticMethod, GetInstanceField, GetStaticField
• GetCached* variants for all of the above
• GetCachedJniType(ref JniType?, ReadOnlySpan<byte>)

Performance

The span overloads use fixed pinning and pass the pointer directly to the JNI function table — zero marshalling. Benchmarked on macOS ARM64 (Release, 1M iterations per operation, NoOp JniObjectReferenceManager):

Operation                       String(ns/op)    UTF8(ns/op)   Speedup
----------------------------------------------------------------------
FindClass                               467            430      1.09x
GetMethodID                             354            297      1.19x
GetStaticMethodID                       341            249      1.37x
GetConstructor                          379            238      1.59x
GetStaticFieldID                        356            257      1.38x
GetInstanceField                        450            364      1.24x

The improvement comes entirely from eliminating the Marshal.StringToCoTaskMemUTF8 + ZeroFreeCoTaskMemUTF8 calls on the native heap — both paths produce identical managed objects (JniMethodInfo / JniFieldInfo).

Test coverage

13 new tests covering:

• Equivalence: string and u8 overloads return identical JNI IDs
• End-to-end: constructor lookup + method invocation via u8
• Cached lookups: GetCached* returns same instance on repeated calls
• Error handling: disposed type, class not found, invalid signature

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