Discrepancy in events for audio vs. subtitle tracks.

[matvp91]

Is your feature request related to a problem? Please describe.

As discussed earlier, writing this down so it doesn't end up in limbo.

From an API perspective, audio and subtitle tracks are quite similar and can both be treated as definitions of a Track.

For audio tracks, specific events indicate when a track is in the process of switching or has switched: Events.AUDIO_TRACK_SWITCHING and Events.AUDIO_TRACK_SWITCHED, respectively. For subtitle tracks, only one event is used: Events.SUBTITLE_TRACK_SWITCH.

There's Events.SUBTITLE_FRAG_PROCESSED too but the same type of event is lacking for audio. I'm not entirely sure if this should be streamlined too, but since we're at it, why not?

Describe the solution you'd like

It makes sense to streamline this behavior and introduce an Events.SUBTITLE_TRACK_SWITCHING too.

Additional context

N/A