Support loading images from AssetManager

[ianhanniballake]

Is your feature request related to a problem? Please describe.
There's a very specific type of Uri on Android in the form of file:///android_asset/your_asset.jpg which, despite its file:// prefix, cannot be opened with ContentResolver.openInputStream() but instead must be opened using AssetManager.

This is true for all Uris in the form of ContentResolver.SCHEME_FILE + ":///android_asset/".

Describe the solution you'd like
I'd like to be able to use:

imageView.load("file:///android_asset/starrynight.jpg")

And have it automatically use AssetManager. I'd also be fine with using the load(Uri) version, although that would be a difference from Picasso and Glide (see examples below).

Additional context
Assets are already supported by Picasso:

 Picasso.get().load("file:///android_asset/starrynight.png").into(imageView)

and Glide

Glide.with(this).load("file:///android_asset/starrynight.jpg").into(imageView)

[colinrtwhite]

Interesting! imageView.load(String) is currently reserved for HTTP URLs, but we should at least support imageView.load(Uri.parse("file:///android_asset/starrynight.jpg")). I think we could make the necessary changes in UriFetcher. I'll make a quick PR.

[ianhanniballake]

load(Uri) support would be great, thanks!

It seems like in other image loading libraries, load(uri: String) = load(Uri.parse(uri)) in all cases and they share the same underlying processing. This ensures that no matter if developers use a String or Uri, the same rules apply, avoiding all cases where developers need to remember to htmlUri.toString() or Uri.parse(contentUriString). Probably worth a separate FR than this as that would be a wider issue than this particular case.

[colinrtwhite]

Good point. Currently, Coil uses url as the named param for load(String) so we'd have to change that to something more general (but hopefully not too general like data; let me know if you have suggestions), but I like the idea of supporting Uris and Urls via load(String).

We could support this by writing a new Mapper or modifying HttpUrlMapper.

[colinrtwhite]

Created a new issue to support parsing Strings as Uris here: #24