[JNIEnv] Support sbyte jagged arrays

[simonrozsival]

Summary

• add explicit sbyte converters beside the existing byte converters used by shared JNIEnv array creation, copying, reading, and element access
• reinterpret sbyte[] as JNI byte-array storage bit-for-bit, so negative managed values retain their two's-complement byte representation
• keep JNI [B fallback routing mapped to managed byte, avoiding any canonicalization of existing byte requests to sbyte

This affects generic and non-generic JNIEnv.NewArray, JNIEnv.GetArray, and JNIEnv.CopyArray paths, including recursive jagged-array handling.

Experiment evidence

Both legacy and trimmable typemap experiments produced the expected JNI signatures ([[B and [[[B) for sbyte jagged arrays. Creation then failed during recursive leaf conversion with:

Don't know how to convert type 'System.SByte' to an Android.Runtime.IJavaObject.

The signatures were therefore already correct; the shared converter dictionaries omitted sbyte while containing byte.

Tests

Added on-device regression coverage for generic and non-generic NewArray/GetArray/CopyArray at ranks 1, 2, and 3:

• sbyte: minimum, negative, zero, and maximum values
• byte: values with high bits set, confirming existing unsigned managed behavior remains unchanged
• JNI signatures asserted as [B, [[B, and [[[B

Validation:

• dotnet build src/Mono.Android/Mono.Android.csproj --no-restore ... — passed (15 existing warnings)
• device tests not run because this worktree has no locally built SDK; direct test-project compilation is blocked by missing Java.Interop reflection test types in the unprepared checkout