fix: improve legacy fairplay workflow detection

[Essk]

Following on from #234 I was made aware of a use case where a user's Fairplay key system was provided as com.apple.fps.1_0 but their custom VJS / contrib-eme setup did not call initLegacyFairplay. The were able to get this working by adding !window.MediaKeys to my modified exit conditional in handleEncryptedEvent.

i.e.:
if (options.keySystems[LEGACY_FAIRPLAY_KEY_SYSTEM] && window.WebKitMediaKeys && !window.MediaKeys)

On modern Safari, this will evaluate to false (as both WebKitMediaKeys and MediaKeys are present), so in their case, handleEncryptedEvent would continue with the standard EME workflow.

This got me thinking that we really should be considering whether the legacy Fairplay workflow is actually in use when deciding whether or not to exit handleEncryptedEvent, as the key system alone is not definite.

So the changes here make that signal more definite by adding a flag in initLegacyFairplay that handleEncryptedEvent can use to know for sure that it should exit.