Expose function to clear all cached images

[wagslane]

A function should be exposed (not a part of the component) to clear all cached images on the local device.

Currently, images are stored in the cache directory (https://docs.expo.io/versions/latest/sdk/filesystem/#filesystemcachedirectory), and they should stay there. However, instead of being stored at the top level, we should make a directory specifically for our own images. Then we can clear out just that directory when we call the function.