Simulator sound has issues in recent Safari versions

[microbit-matt-hillsdon]

In Safari the audio context never leaves the "suspended" state. We only write one frame's worth of data and are never asked for more. This definitely used to work and continues to work on other browsers. Trying old versions of the sim demonstrates that they also don't work so likely the browser has changed.

In the simulator demos it works on second attempt (stop/play). In the Python Editor this doesn't work.

We're supposed to create or resume the audio context from a user interaction. We should try to put an audio context resume call in the event handler itself to make it obviously correct. Currently we post a message requesting a flash and don't touch the context until we get a reply.