TTML parser doesn't support namespaces

[TheModMaker]

Have you read the FAQ and checked for duplicate open issues?:
Yes
What version of Shaka Player are you using?:
v2.4.4
Can you reproduce the issue with our latest release version?:
Yes
Can you reproduce the issue with the latest code from master?:
Yes
Are you using the demo app or your own custom app?:
Demo app
If custom app, can you reproduce the issue using our demo app?:

What browser and OS are you using?:
Any
What are the manifest and license server URIs?:
https://d1chyo78gdexn4.cloudfront.net/vualto-demo/elephants-dream/elephants-dream_nodrm.ism/manifest.mpd

What did you do?
Play TTML asset that uses XML namespaces with different prefixes than normal.
What did you expect to happen?
Play text fine.
What actually happened?
Cannot find attributes since the TTML parser looks up by simple name instead of namespace-qualified.

Found while investigating #1584.

[joeyparrish]

Fix cherry-picked to v2.4.5.