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.

Ask the activity INSTANCE, not its Class: Context.getClassLoader() is
documented to return a loader for the application package
unconditionally, whereas Class.getClassLoader() returns null for classes
loaded by the bootstrap loader.

Describe and clear the pending Java exception on every error path. jni
0.21 leaves the exception pending when a call returns Err, and the next
JNI call then aborts the process with 'JNI DETECTED ERROR IN
APPLICATION: ... called with pending exception' -- which is what turns a
recoverable ClassNotFoundException into an unreadable ART abort.

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))),