TTML: Support tts:lineHeight setting

### [REQUIRED] Use case description
Configurable Subtitles line spacing (extra, multiplier)
Business requirement is that lines needed to be further apart (based on provided line spacing). With ExoPlayer it can be achieved currently by applying lineSpacingMultiplier to Activity theme, but it seems to be not the best approach, as it introduces global change.

### Proposed solution
Currently line spacing logic is handled in SubtitlePainter - **lineSpacingExtra** and **lineSpacingMultiplier** is taken from Activity theme. Consider adding method to set expected values with SubtitleView, or add it to CaptionStyleCompat?

### Alternatives considered
n/a