Fix sideload text track issues on iOS

[cobarx]

This PR is designed to fix #1155.

While investigating this, I discovered the code would cause text tracks to be displayed even if the type was set to "disabled". I've refactored the code so that we only look for text tracks matching the device language if the type is "system".

[cobarx]

@ashnfb I can't figure out how to make it so all text tracks get disabled. The code to loop through all the tracks and update the enabled status works fine as long as we enable at least one track. But if I setEnabled:NO for all of them, the text for the most recently selected track gets stuck on the screen and never disappears. Any ideas how we can tackle this? I've looked all over Stack Overflow and can't find anything.

Here's the relevant code:

  for (int i = firstTextIndex; i < _player.currentItem.tracks.count; ++i) {
    BOOL isEnabled = NO;
    if (selectedTrackIndex != RCTVideoUnset) {
      isEnabled = i == selectedTrackIndex + firstTextIndex;
    }
    [_player.currentItem.tracks[i] setEnabled:isEnabled];
  }

[cobarx]

I posted a Stack Overflow question to see if I can get some info on how to tackle this:
https://stackoverflow.com/questions/51585932/how-do-you-disable-all-text-tracks-in-an-avmutablecomposition

[ashnfb]

@cobarx going to have a look at these issues today and tomorrow, plus the onLoad changes suggested

[cobarx]

@ashnfb Ok, thanks man. I've asked a friend who's pretty familiar with AVPlayer how to tackle this, but I haven't heard back from him yet.

[cvaldezscse]

@cobarx Does someone looking at this issue? i can help you if you tell me where to start from