[monodroid] AndroidEnablePreloadAssemblies causes runtime crash.

Fixes dotnet#5838

Turning off Assembly preloading results in the following runtime error

```
android.runtime.JavaProxyThrowable: System.NullReferenceException: Object reference not set to an instance of an object
    at MyLibrary.MainActivity.OnCreate(Bundle bundle)
    at Android.App.Activity.n_OnCreate_Landroid_os_Bundle_(IntPtr jnienv, IntPtr native__this, IntPtr native_savedInstanceState)
    at com.xamarin.classlibrarymainlauncherruns.MainActivity.n_onCreate(Native Method)
    at com.xamarin.classlibrarymainlauncherruns.MainActivity.onCreate(MainActivity.java:29)
```

This only occurs on one unit test [`DebuggingTest.ClassLibraryMainLauncherRuns()`](https://github.com/xamarin/xamarin-android/blob/bf4f4f42af26cdddb46087deed59aae8424f7942/tests/MSBuildDeviceIntegration/Tests/DebuggingTest.cs#L74-L124)
This happens because the `MainActivity` in that test is in a library project.
As a result when we do not preload assemblies the main assembly is NEVER
loaded. This means `ResourceIdManager.UpdateIdValues()` never finds the
`Resource` class from the main assembly, and as such none of the ids are
updated.

To fix this , when running without assembly preloading we will ALWAYS
load the first assembly in the assembly list. This will always be the
main assembly and will contain the `Resource` class.

src/monodroid/jni/monodroid-glue-internal.hh

@@ -224,7 +224,7 @@ namespace xamarin::android::internal
 		void disable_external_signal_handlers ();
 		void lookup_bridge_info (MonoDomain *domain, MonoImage *image, const OSBridge::MonoJavaGCBridgeType *type, OSBridge::MonoJavaGCBridgeInfo *info);
 		void load_assembly (MonoDomain *domain, jstring_wrapper &assembly);
-		void load_assemblies (MonoDomain *domain, jstring_array_wrapper &assemblies);
+		void load_assemblies (MonoDomain *domain, bool preload, jstring_array_wrapper &assemblies);
 		void set_debug_options ();
 		void parse_gdb_options ();
 		void mono_runtime_init (dynamic_local_string<PROPERTY_VALUE_BUFFER_LEN>& runtime_args);

src/monodroid/jni/monodroid-glue.cc

@@ -1595,7 +1595,7 @@ MonodroidRuntime::load_assembly (MonoDomain *domain, jstring_wrapper &assembly)
 }
 
 inline void
-MonodroidRuntime::load_assemblies (MonoDomain *domain, jstring_array_wrapper &assemblies)
+MonodroidRuntime::load_assemblies (MonoDomain *domain, bool preload, jstring_array_wrapper &assemblies)
 {
 	timing_period total_time;
 	if (XA_UNLIKELY (utils.should_log (LOG_TIMING)))
@@ -1604,6 +1604,9 @@ MonodroidRuntime::load_assemblies (MonoDomain *domain, jstring_array_wrapper &as
 	for (size_t i = 0; i < assemblies.get_length (); ++i) {
 		jstring_wrapper &assembly = assemblies [i];
 		load_assembly (domain, assembly);
+		// only load the first "main" assembly if we are not preloading.
+		if (!preload)
+			break;
 	}
 
 	if (XA_UNLIKELY (utils.should_log (LOG_TIMING))) {
@@ -1638,8 +1641,8 @@ MonodroidRuntime::create_and_initialize_domain (JNIEnv* env, jclass runtimeClass
 	if (assembliesBytes != nullptr)
 		designerAssemblies.add_or_update_from_java (domain, env, assemblies, assembliesBytes, assembliesPaths);
 #endif
-	if (androidSystem.is_assembly_preload_enabled () || (is_running_on_desktop && force_preload_assemblies))
-		load_assemblies (domain, assemblies);
+	bool preload = (androidSystem.is_assembly_preload_enabled () || (is_running_on_desktop && force_preload_assemblies));
+	load_assemblies (domain, preload, assemblies);
 	init_android_runtime (domain, env, runtimeClass, loader);
 
 	osBridge.add_monodroid_domain (domain);