The tts:textAlign property only applies to <p> elements, which
correspond 1:1 with ExoPlayer Cue objects, so we can use
Cue.textAlignment to store this info instead of encoding it in
the span-styled text.
This will mean that TTML subtitles used with
SubtitleView#setApplyEmbeddedStyles(false) will start respecting
the tts:textAlign properties from the source data (currently this
information is stripped when we remove all span styling). I think this
is working-as-intended, we respect alignment of other subtitle types
(e.g. WebVTT) when applyEmbeddedStyles=false. We also respect all other
'positioning' related properties in this case e.g. Cue.position and
Cue.line.
PiperOrigin-RevId: 310895499
