Make `asset_manager` configurable

[jinleili]

What problem does this solve or what need does it fill?

Integrating the Bevy engine into an existing app may be a common scenario, but in this scenario it is difficult for the user to manually initialize the ndk-glue because the ANativeActivity parameter cannot be provided and the ndk-glue is not initialized which causes AndroidAssetIo::load_path panic.

What solution would you like?

Since ANativeActivity is only used in bevy to get asset_manager:

let asset_manager = ndk_glue::native_activity().asset_manager()

Allowing asset_manager to be made available through configuration would give users more freedom.

Additional context

Scenario demo: bevy-in-app

[mockersf]

ndk-glue will be removed in #6830

[jinleili]

@mockersf Thank you for the link, I learned how to customize AssetIoPlugin from #6830, now solved the configuration issue of asset_manager.