* Create generic stream description classes to represent
Representations, AdaptationSets, and Periods.
* Move generic stream selection logic out of MpdProcessor and
into StreamInfoProcessor.
* Refactor DashVideoSource and DashStream to use the generic
stream description classes.
A follow-up patch is required to re-organize files and namespaces.
b/18903621
b/18948677
Change-Id: I8bbb87ebf0242b31541199d0d1d3c40767c93a29