[NativeAOT] Initialize GC threshold much earlier

[grendello]

The threshold value is used by the global reference creation routine to determine whether to start GC collection:

if (gc >= JNIEnvInit.gref_gc_threshold) {
  Logger.Log (LogLevel.Debug, "monodroid-gc", gc + " outstanding GREFs. Performing a full GC!");
  System.GC.Collect ();
}

However, the threshold value was being initialized after we've already invoked some non-trivial managed code in the managed NativeAOT runtime, too late to avoid at least a handful of System.GC.Collect () calls:

09-15 14:37:25.923 10761 10761 D monodroid-gc: 1 outstanding GREFs. Performing a full GC!
09-15 14:37:25.923 10761 10761 D monodroid-gc: 2 outstanding GREFs. Performing a full GC!
09-15 14:37:25.923 10761 10761 D monodroid-gc: 3 outstanding GREFs. Performing a full GC!
09-15 14:37:25.924 10761 10761 D monodroid-gc: 3 outstanding GREFs. Performing a full GC!

Fix this by splitting the GC threshold value initialization into a separate method which is invoked as soon as possible from the JNI_OnLoad function exported by the NativeAOT application.

[Copilot]

Pull Request Overview

This PR fixes a timing issue in NativeAOT where the GC threshold was being initialized too late, causing unnecessary garbage collection calls during early runtime initialization.

• Splits GC threshold initialization into a separate method called earlier in the process
• Adds explicit early initialization call in NativeAOT's JNI_OnLoad function
• Removes redundant GC threshold initialization from the main InitializeJniRuntime method

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Mono.Android/Android.Runtime/JNIEnvInit.cs	Extracts GC threshold initialization into new NativeAotInitializeMaxGrefGet method and removes it from InitializeJniRuntime
src/Microsoft.Android.Runtime.NativeAOT/Android.Runtime.NativeAOT/JavaInteropRuntime.cs	Calls the new GC threshold initialization method early in JNI_OnLoad before other initialization

[filipnavara]

Tested on our app, the inital 4 GCs with "outstanding GREFs. Performing a full GC!" message are gone.

[jonathanpeppers]

I'm rerunning these, but if they end up passing, this looks good:

There are some other test failures caused by Android 36.1 changes, hopefully I can fix shortly.