DASH-IF IOP 3.0 Compliance: Move ContentProtection to AdaptationSet element.

[kqyang]

7.5.2. MPD Content Protections Constraints

For an encrypted Adaptation Set, ContentProtection Descriptors SHALL always be present in the AdaptationSet element, and apply to all contained Representations.

We currently put ContentProtection element in Representation but not in AdaptationSet.

This issue is related to issue #24 and issue #25. Representations encrypted using different keys need to go into different adaptation sets. We should also use the AdaptationSet@group attribute to indicate that switching between these AdaptationSets is allowed."

[kqyang]

Fixed in 869d7f4. The feature is controlled by flag --generate_dash_if_iop_compliant_mpd, which is not enabled by default yet.