SDK / exoplayer

Currently, DashMediaPeriod only takes into account as CEA-608 accessibility tags as embedded
closed captions tracks CEA-608. CEA-708 closed captions format is parsed when is present on
its own AdaptationSet, but not when is embedded as an accessibility tag in a video AdaptaticonSet.

Embedded CEA-708 support is added by parsing accessibility tags like the example below:

  <Accessibility schemeIdUri="urn:scte:dash:cc:cea-708:2015" value="1=lang:eng;2=lang:deu"/>
  <Accessibility schemeIdUri="urn:scte:dash:cc:cea-708:2015" value="1=lang:eng;2=lang:eng,war:1,er:1"/>

so it creates a new CEA-708 track for accessibility tags with schemeIdUri = urn:scte:dash:cc:cea-708:2015
and extract accessibilityChannel and language from value attribute.

Signed-off-by: Jorge Ruesga <jorge@ruesga.com>