Extract encryption related handlings from Fragmenter and move into new
class EncrpytingFragmenter.
Change-Id: Id42493cd321fd7e306f8ce522c7ff04990965256
This CL also includes two related changes:
1. Support serialization of raw PSSH box;
2. Refactor segmenter interface a bit.
Change-Id: I668aa01a5a5ff524ad6fe74976339a3fd499120a