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))),