Play movies has an Allocator that attempts to allocate a single
huge byte[] up front to minimize the risk of GC pauses. This abstraction
will be required to keep that when updating them to the new Exo.
The key change here is that nextLoadPositionUs is set to -1
if we're not loading but don't have a next chunk ready to
load. This ensures that "missing chunks" in one stream don't
prevent chunks in another stream from loading. This occurs
in SmoothStreaming with TTML subtitles, where the chunks are
sparse.
