fix: auto-removal remote text tracks being removed when not supposed to

[gkatsev]

Description

We added a feature so that remote text tracks can auto-removed when a source changes. However, in 6.x we changed the source behavior to be asynchronous meaning that some text tracks were accidentally being removed when they weren't supposed to be.
For example:

var player = videojs('my-player');
player.src({src: 'video.mp4', type: 'video/mp4'});
 // set second arg to false so that they get auto-removed on source change
player.addRemoteTextTrack({kind: 'captions', src: 'text.vtt', srclang: 'en'}, false);

Now when the player loads, this captions track is actually missing because it was removed.

Specific Changes proposed

Instead of adding auto-removal tracks immediately to the list, wait until we've selected a source before adding them in.
This probably needs a test.

Fixes #4403 and #4315.

[gkatsev]

#4434 was already approved and there have not been any changes here.