Fix crash on attaching metadata output

[Fonos-development]

The issue: doublesymmetry/react-native-track-player#1818
The previous fix: #68

The previous attempt to fix it was not ideal cause we don't know the exact observation removal time.

My approach:

• Deferred Addition of Metadata Output
• Each observed item now gets a fresh instance of metadata output for cleaner management.
• Added a check to ensure safe removal of metadata output from the observing item.

[dcvz]

I've left some comments / thoughts. Overall looks good to me though, thanks for this @Fonos-development

[dcvz]

We can also remove the private let metadataOutput = AVPlayerItemMetadataOutput() above right?

[dcvz]

I wonder whether we need to add a check in the delegate method func metadataOutput() to make sure we're receiving metadata from the "currently focused' item and output before we broadcast it. We could still do it by keeping a reference to the latest created output currentMetadataOutput and comparing the output given in the delegate to it.

I think once we reassign currentMetadataOutput any old metadata outputs will keep hanging in memory until we call remove(metadataOutput) in the stopObservingCurrentItem. We should probably do a small check on that.

[Fonos-development]

Updated, thanks for your advice. Kindly have a look at the new commit.

[dcvz]

This is clever. We could probably put this in an extension and remove all outputs for safety.

extension AVPlayerItem {
    func removeAllMetadataOutputs() {
        for output in self.outputs.filter({ $0 is AVPlayerItemMetadataOutput }) {
            self.remove(output)
        }
    }
}

[dcvz]

LGTM 👍 thanks for this @Fonos-development