Support loading images from AssetManager.

[colinrtwhite]

Fixes #10.

This was proposed as an enhancement, but is more of a bugfix. There was a test covering asset manager loading, but it used Robolectric. Let's make it an instrumentation test instead.

[Jawnnypoo]

LGTM @ianhanniballake any additional feedback on this?

[ianhanniballake]

Overall, LGTM.

i think personally I might have preferred a separate AssetFetcher that would have a handles() that specifically calls out this special case (updating the handles of UriFetcher to exclude asset Uris if it isn't possible to guarantee ordering between the two) rather than complicate the logic of UriFetcher itself, but from an external developer perspective, the chosen approach does fix the issue and future restructuring can be done without changing the external API surface / behavior if deemed useful.

[colinrtwhite]

@ianhanniballake Good point. I think I prefer this approach because it avoids creating a new Fetcher for a "special" Uri case, but we can always move this logic into a separate Fetcher without changing the API if it becomes too unwieldily.

[ianhanniballake]

Makes sense to me.