fix(ttml): Center subtitles by default (#4023)

Closes: #4015

Author: Álvaro Velad Galván

lib/text/ttml_text_parser.js

@@ -544,8 +544,10 @@ shaka.text.TtmlTextParser = class {
 
       cue.textAlign = Cue.textAlign[align.toUpperCase()];
     } else {
-      // Default value is START: https://bit.ly/32OGmvo
-      cue.textAlign = Cue.textAlign.START;
+      // Default value is START in the TTML spec: https://bit.ly/32OGmvo
+      // But to make the subtitle render consitent with other players and the
+      // shaka.text.Cue we use CENTER
+      cue.textAlign = Cue.textAlign.CENTER;
     }
 
     const displayAlign = TtmlTextParser.getStyleAttribute_(