Mobile endpoints are flaky with the WebSpeechAPI:
- iOS versions that support it are borking our outbound audio when it's
enabled
- Android speech recognition has flaky locale detection and speech
transcription
Additionally: the support check is not checking the WebSpeechAPI
availability properly, so older devices (eg iOS 12) are flagged as
supported even though they aren't.
This commit adds a configuration flag (public.audioCaptions.mobile) to
control transcription availability on mobile. False by default.
Also extends the setSpeechVoices support check and
hasSpeechRecognitionSupport method to prevent false positives.
6c8b097eba