Add support for NativeScript 6, and playback inside modals on Android

[EddyVerbruggen]

Hey @triniwiz,

I was trying to show a video inside a modal in a NativeScript 6 app and found that it didn't work for two reasons:

1. com.google.android.youtube.player.YouTubePlayerFragment extends android.app.Fragment directly, which means it's not compatible with AndroidX (and thus NativeScript 6). However, com.google.android.youtube.player.YouTubePlayerSupportFragment extends android.support.v4.app.Fragment which is.
2. As mentioned in No View found error #40 the plugin can't resolve the correct view, but by using the right fragment manager we can get around this. It's a private function in NativeScript's core modules, but it seemed like the easiest way to fix this and I don't think it will be removed any time soon (or ever actually).

In case you accept this PR, please also publish a new version to npm because I'm sure more N6-using folks will encounter this issue. A major version bump would be a good idea because I'm not 100% sure it's backward compatible (didn't test it with N5).

Cheers,
Eddy

[EddyVerbruggen]

Hey @triniwiz would you please consider merging this? I don't mind if not, then I'll know I'll have to ship it some other way in my app. Thx!