[AdaptiveTree] On parsers use "und" language code for unknown cases

[CastagnaIT]

Description

"unk" uses is not really handled by kodi core so make conversion to fails by showing "unknown",
instead "und" lang code is expected by kodi core for "undetermined" language

On SmoothStreaming specs "und" language code is expected for unknown/neutral language on the Language tag
On DASH specs its not really defined, but shown on the manifest examples on AdaptationSet lang tag
On HLS its not considered as possible case for unknown, i think because the LANGUAGE tag is optional

Motivation and context

im doing changes/cleanups on kodi core

How has this been tested?

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• I have read the Contributing document
• My code follows the Code Guidelines of this project
• My change requires a change to the Wiki documentation
• I have updated the documentation accordingly