manganis: load FFI classes from the activity classloader, not FindClass

JNIEnv::find_class resolves against the system classloader on a thread
attached from native code, which cannot see APK classes (#5629). Route
the generated #[manganis::ffi] type lookup through
load_class_from_classloader, and reimplement that helper to ask the
activity's own ClassLoader.loadClass instead of Class.forName.

packages/manganis/manganis-macro/src/ffi.rs

@@ -398,7 +398,7 @@ impl FfiBridgeParser {
                         // Use with_activity which returns Option<R>, wrapping our Result in the Option
                         let inner_result: Option<Result<Self, String>> = manganis::android::with_activity(|mut env, activity| {
                             // Find the class
-                            let class_result = env.find_class(#class_name_lit);
+                            let class_result = manganis::android::load_class_from_classloader(env, activity, #class_name_lit);
                             let class = match class_result {
                                 Ok(c) => c,
                                 Err(e) => return Some(Err(format!("Failed to find class {}: {:?}", #class_name_lit, e))),

packages/manganis/manganis/src/android/java.rs

@@ -111,15 +111,34 @@ pub fn request_permissions_via_helper(
     Ok(())
 }
 
-/// Load a Java class from the APK's classloader
+/// Load a Java class from the APK's classloader.
+///
+/// `JNIEnv::find_class` resolves against the *system* classloader on a thread that
+/// was attached from native code (`JavaVM::attach_current_thread`), which cannot see
+/// classes packaged in the APK, so it raises `ClassNotFoundException`. The activity's
+/// own classloader can, so we ask the activity for it and call `loadClass` on it.
+///
+/// `class_name` may be given in either JNI form (`com/example/Foo`) or binary form
+/// (`com.example.Foo`); `loadClass` requires the binary form, so slashes are converted.
 pub fn load_class_from_classloader<'env>(
     env: &mut JNIEnv<'env>,
+    activity: &JObject<'_>,
     class_name: &str,
 ) -> Result<JClass<'env>> {
-    let class_name_jstring = new_string(env, class_name)?;
-    let class = env.call_static_method(
-        "java/lang/Class",
-        "forName",
+    let binary_name = class_name.replace('/', ".");
+    let activity_class = env.get_object_class(activity)?;
+    let loader = env
+        .call_method(
+            &activity_class,
+            "getClassLoader",
+            "()Ljava/lang/ClassLoader;",
+            &[],
+        )?
+        .l()?;
+    let class_name_jstring = new_string(env, &binary_name)?;
+    let class = env.call_method(
+        &loader,
+        "loadClass",
         "(Ljava/lang/String;)Ljava/lang/Class;",
         &[JValue::Object(&class_name_jstring)],
     )?;