Convert SpeechSynthesisAnnouncer.initialized to a Property

[zepumph]

@chrisklus and I ran into a barrier here while working on https://github.com/phetsims/number-play/issues/188, and a notification when this changes would be very helpful.

tagging our main man @jessegreenberg so he is aware.

[chrisklus]

Thanks! Committed above, closing.