Fetch local files asynchronously.

This patch implements a new fetcher that only handles local files.
While libcurl supports file:// urls, the stream can't be suspended when
accessing local files.

This new FileFetcher is based on the brillo::FileStream class which
properly handles the asynchronous reads from regular files.

Bug: 28866512
TEST=Added unittest. Deployed an update from a file:// URL.

Change-Id: Ie9d07dda2d773312e55be3c0ab7c9e5b737be18b