SDK / exoplayer

When debugging and fixing Issue: google/ExoPlayer#10666 I wanted to write a regression
test, but needed to add a test first... This is just a small bit of
coverage to start with. It checks the field/channel filtering works
correctly, but doesn't check any styling info. It also doesn't test
'pop on' subtitles (i.e. when the subtitle isn't shown until a 'end of
subtitle' signal is received).

PiperOrigin-RevId: 480644568