- ContentProtection elements should be added explicitly by the
AddContentProtection() methods.
- This is because some MPDs should have ContentProtection at
AdaptationSet level instead of Representation.
- Change SimpleMpdNotifier, which uses MpdBuilder, to add
ContentProtectionElements. The logic is moved from MuxerListener.
- Add Element class for specifying subelements for
ContentProtectionElement.
Change-Id: I9bedfb3e5a5ac0b3d5c702f1e6e4a8608c978d1d