Modify behavior of DownloadResumable to match other FileSystem operations. (#958)
Modify behavior of DownloadResumable to match other FileSystem operations. (#958)

Addresses the following issues:
https://github.com/expo/universe/issues/939
https://github.com/expo/universe/issues/938

- This PR modifies the behavior of `DownloadResumable` to match other `FileSystem` operations.  If the user initiates the download of a remote file to a local uri which already exists, the existing file will be overwritten.
- A TODO from the following PR was addressed: https://github.com/expo/universe/pull/957
```// TODO: Fix. iOS sends UUID and Android sends uuid.```
- Documentation was updated to reflect behavior and the example was expanded to include resuming a download across app restarts.

fbshipit-source-id: 914ae7f

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