To encourage the use of accounts, new devices should be presented with
the login screen where they can create an account. New users can still
switch to anonymous login if they choose.
- Dictation reliable raises the handleAudioInterrupt notification, so that
is used for disabling / enabling audio
- Siri sometimes doesn't raise the handleAudioInterrupt notification, so
rely on applicationDidBecomeActive / applicationWillResignActive.
- Trial and error showed it is best to destroy and re-create the OS audio
objects when audio is disabled / enabled.
