AssetsLibraryIconProvider should use AssetImageProvider to get icons for image types it supports

Currently AssetsLibraryIconProvider is synchronous, generates icons from full size asset images, and caches generated images locally. Generating icons from full size asset images is wasteful, as AssetImageProvider already does that and persistently caches the results. So AssetsLibraryIconProvider should utilize AssetImageProvider for generating icons for asset types it supports. Or preferably the two should just be merged into a single generic image provider for all assets. Note that AssetImageProvider is asynchronous and AssetsLibraryIconProvider is synchronous, so this may require some refactoring of things.