SDK / exoplayer

Using a file:// URL for loading an HLS stream (for testing) would fail due to
casting the connection to an HttpURLConnection in DefaultHttpDataSource.

This change makes UriDataSource an interface for objects that are DataSources
with URIs. That allows for reading manifests for HLS using just a UriDataSource
rather than an HttpDataSource (URLs in the playlist are relative to the data
source's URL so the getUri method is needed).