dash: Store entire ContentProtection node in protection event data

Some manifests use the ContentProtection node to store additional information such as the license server url. Our MPD parser used to process the ContentProtection node, extracting Playready PSSH boxes. However for other DRM systems, only the value attribute was passed down to the protection event, so for example, Widevine data was not parsed at all and "Widevine" was passed to the event, which is not very useful for decryptors that require a PSSH init data.

Parsing should now be done by decryptors which will receive the entire ContentProtection XML node as a string. This gives more "freedom" to the decryptor which can then detect and parse custom nodes as well.