For mpd codes, rename dash_packager to edash_packager; for media codes,
add namespace edash_packager.
Change-Id: I1120e6110c734cd02bdbe15f1cf7abddcdfdd7ea
This CL also includes two related changes:
1. Support serialization of raw PSSH box;
2. Refactor segmenter interface a bit.
Change-Id: I668aa01a5a5ff524ad6fe74976339a3fd499120a