[Java.Interop] Avoid locks for cached JNI member lookups

[jonathanpeppers]

Description

JniPeerMembers cached JNI method and field IDs in dictionaries protected by
monitor locks. Every generated binding invocation paid that lock cost even
after the JNI member had already been resolved.

Use ConcurrentDictionary for the instance/static method and field caches,
plus subclass constructor dispatch. Cached reads no longer enter a monitor,
while first field resolution remains serialized and existing method
replacement, fallback, exception, and lifetime behavior is preserved.

This also adds an instrumentation-only Android BenchmarkDotNet app for
measuring JNI invocation changes on-device.

Related to #11885.

Performance

Pixel 5, .NET 11 CoreCLR:

Path	Before	After	Change
Cached virtual JNI invocation	221.4 ns	180.7 ns	-18%

Pixel 5 MAUI/CoreCLR cold startup, 40 counterbalanced pairs built from
identical restored packages:

APK	Mean startup
Base	835.62 ms
Changed	832.85 ms

Paired changed-minus-base delta: -2.77 ms, with a 95% confidence
interval of -7.61 to +2.06 ms.

apkdiff reports:

Metric	Change
APK size	0 bytes (0.00%)
Shared libraries	-1,952 bytes (-0.01%)

Testing

• Java.Interop managed build

• JniPeerMembersTests: 8 passed, 1 skipped

• Android BenchmarkDotNet instrumentation run

• 40-pair counterbalanced MAUI/CoreCLR cold-start comparison

• apkdiff package comparison

• Useful description of why the change is necessary

• Links to related issues

• Unit tests

[Copilot]

Pull request overview

This PR improves Java.Interop JNI member lookup performance by removing monitor-lock overhead from cached method/field ID access in JniPeerMembers, and adds an Android on-device BenchmarkDotNet instrumentation app to measure JNI invocation changes.

Changes:

• Replace lock-protected Dictionary caches with ConcurrentDictionary caches for JNI instance/static method and field lookups.
• Preserve serialized first-time field resolution while making cached reads lock-free.
• Add a new instrumentation-only Android BenchmarkDotNet app and wire it into the test solution.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Xamarin.Android-Tests.slnx	Adds the new Android benchmarks project to the tests solution.
tests/Android.Benchmarks/README.md	Documents how to run the on-device BenchmarkDotNet instrumentation benchmarks.
tests/Android.Benchmarks/JniMethodInfoBenchmarks.cs	Adds benchmarks comparing cached JNI invocation vs generated binding calls.
tests/Android.Benchmarks/BenchmarkInstrumentation.cs	Implements the Android instrumentation entry point that runs BenchmarkDotNet in-process.
tests/Android.Benchmarks/AndroidManifest.xml	Provides a minimal manifest for the instrumentation-only benchmark app.
tests/Android.Benchmarks/Android.Benchmarks.csproj	Defines the Android benchmark app project and BenchmarkDotNet dependency.
external/Java.Interop/tests/Java.Interop-Tests/Java.Interop/JniPeerMembersTests.cs	Updates test reflection expectations to match ConcurrentDictionary caches.
external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniStaticMethods.cs	Switches static method cache to ConcurrentDictionary and removes monitor locking on cached reads.
external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniStaticFields.cs	Switches static field cache to ConcurrentDictionary and adjusts the synchronization strategy.
external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceMethods.cs	Switches instance method + subclass-constructor caches to ConcurrentDictionary and removes monitor locking on cached reads.
external/Java.Interop/src/Java.Interop/Java.Interop/JniPeerMembers.JniInstanceFields.cs	Switches instance field cache to ConcurrentDictionary and adjusts the synchronization strategy.