[android] Don't include JNI_OnLoad in libSystem.Security.Cryptography.Native.Android.a (#103231)

I'm working on a feature in .NET For Android which will allow us to link
all the native bits into a single shared library at application build
time.  In order to make it possible, no archive (`.a`) may contain any
`JNI_OnLoad` functions (called by `JavaVM` when initializing a Java
extension DSO), because our runtime already contains one and there Can
be Only One(tm).

`libSystem.Security.Cryptography.Native.Android` is currently the only
BCL support native library which contains `JNI_OnLoad` and thus it
prevents us from linking it into our runtime.  This PR changes things
a bit my moving the initialization code to a separate
function (`AndroidCryptoNative_InitLibraryOnLoad `) which remains in the `.a` archive and
can be called by `.NET For Android` runtime from its own `JNI_OnLoad` as
well as by the `libSystem.Security.Cryptography.Native.Android.so` from
its `JNI_OnLoad`, which this PR moves to a separate source file that is
compiled only into the shared version of the crypto support library.

src/native/libs/System.Security.Cryptography.Native.Android/CMakeLists.txt

@@ -33,7 +33,7 @@ set(NATIVECRYPTO_SOURCES
 
 add_library(System.Security.Cryptography.Native.Android
     SHARED
-    ${NATIVECRYPTO_SOURCES}
+    ${NATIVECRYPTO_SOURCES} pal_jni_onload.c
     ${VERSION_FILE_PATH}
 )
 

src/native/libs/System.Security.Cryptography.Native.Android/pal_jni.c

@@ -689,11 +689,10 @@ int GetEnumAsInt(JNIEnv *env, jobject enumObj)
     return value;
 }
 
-JNIEXPORT jint JNICALL
-JNI_OnLoad(JavaVM *vm, void *reserved)
+jint AndroidCryptoNative_InitLibraryOnLoad (JavaVM *vm, void *reserved)
 {
     (void)reserved;
-    LOG_INFO("JNI_OnLoad in pal_jni.c");
+    LOG_DEBUG("%s in %s", __PRETTY_FUNCTION__, __FILE__);
     gJvm = vm;
 
     JNIEnv* env = GetJNIEnv();

src/native/libs/System.Security.Cryptography.Native.Android/pal_jni.h

@@ -605,6 +605,11 @@ jfieldID GetField(JNIEnv *env, bool isStatic, jclass klass, const char* name, co
 jfieldID GetOptionalField(JNIEnv *env, bool isStatic, jclass klass, const char* name, const char* sig) ARGS_NON_NULL_ALL;
 JNIEnv* GetJNIEnv(void);
 
+// This is supposed to be called by embedders who link the **static** archive of this library.
+// The function must be called from the embedder's `JNI_OnLoad` function prior to using any
+// APIs in this library.
+jint AndroidCryptoNative_InitLibraryOnLoad (JavaVM *vm, void *reserved);
+
 int GetEnumAsInt(JNIEnv *env, jobject enumObj) ARGS_NON_NULL_ALL;
 
 void* xmalloc (size_t size) __mallocfunc __BIONIC_ALLOC_SIZE(1) __wur;

src/native/libs/System.Security.Cryptography.Native.Android/pal_jni_onload.c

@@ -0,0 +1,10 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#include "pal_jni.h"
+
+JNIEXPORT jint JNICALL
+JNI_OnLoad(JavaVM *vm, void *reserved)
+{
+	return AndroidCryptoNative_InitLibraryOnLoad (vm, reserved);
+}