Add directory related functions to AndroidAssetReader

[esensar]

Objective

• Fixes directories support on android #9968

Solution

• Uses open_dir to read directories and collects child list, since it can't be shared across threads.
• For is_directory, uses result of open, which will fail for directories. I tried using the result of open_dir for this, but it was successful for files too, which made loading folders return empty lists, since open_dir was successful and treated all files as empty directories.
• Ignoring meta files was copied from filesystem implementation

---

Changelog

• Fixed: Android's AssetReader implementation now supports read_directory and is_directory.

Notes

I noticed late that there was the #9968 issue (I only noticed #9591), so I have also missed that a PR was already open (#9969). Feel free to copy over the fixes from this one over there.
The only difference I notice between these 2, is that I have used open instead of open_dir for is_directory implementation. I have tried with open_dir too, but unfortunately that didn't work. I tested this on an actual device, using the mobile example, by making some minor changes:

#[derive(Resource)]
struct FolderAssets(Handle<LoadedFolder>);

// the `bevy_main` proc_macro generates the required boilerplate for iOS and Android
#[bevy_main]
fn main() {
    // ...
    .add_systems(Startup, (setup_scene, load_music_files))
    .add_systems(
        Update,
        // Removed the handle_lifetime since AudioBundle is added later
        (touch_camera, button_handler, setup_music),
    );
   // ...
}

fn load_music_files(asset_server: Res<AssetServer>, mut commands: Commands) {
    let sounds = asset_server.load_folder("sounds");
    commands.insert_resource(FolderAssets(sounds));
}

fn setup_music(
    mut commands: Commands,
    folders: Res<Assets<LoadedFolder>>,
    mut loaded_event: EventReader<AssetEvent<LoadedFolder>>,
) {
    for event in loaded_event.read() {
        if let AssetEvent::LoadedWithDependencies { id } = event {
            if let Some(folder) = folders.get(*id) {
                warn!("Folder items: {:?}", folder.handles);
                if let Some(source) = folder.handles.first() {
                    commands.spawn(AudioBundle {
                        source: source.clone().typed::<AudioSource>(),
                        settings: PlaybackSettings::LOOP,
                    });
                }
            }
        }
    }
}

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[Kanabenki]

LGTM!

@esensar seems like I can't resolve the remaining threads from my previous review for some reason, but you should be able to.

[kristoff3r]

I haven't tested it myself but given the constraints the code seems reasonable.

[esensar]

@mockersf Is this alright to merge? Sorry, I haven't checked it in a while and I saw this is still open today.

[mockersf]

didn't test, but the code seems fine

[mockersf]

@esensar thanks for the ping! sorry I let this PR slip through.

The android build is failing, but should be an easy fix (just avoiding shadowing a variable, as in my suggestion)

[BenjaminBrienen]

Fair enough

[BenjaminBrienen]

.as_asset_file() or something could probably be its own function since that seems useful to have generally speaking.

[esensar]

I agree, but I think it might be better to just extract out the "meta" file detection and filtering. I am worried that something like .as_asset_file() might be confusing (since it would take a file path and then either return Some or None, depending on this condition - and that Some value would be the original value unchanged).

Do you think that would make sense?

[BenjaminBrienen]

Can we add this expectation to the documentation?

[esensar]

This is a message that is also used in bevy_winit, so it is just reused here. I agree it would be good to add to the docs. I can add it separately to the android page in the book (which is currently hidden, but I guess it is good to have it there so it is not forgotten once it gets published).