'echoCancellation' should be disabled by 'disableAEC'
'disableAP' should disable all options

As new use case for joining a conference without calling GUM has been
added to jitsi-meet enumerate devices should not be delayed. In fact not
only it was delayed, but never called in such case
(getUserMediaStatus.initialized stays false forever).

* make audio processing constraints configurable

AP: echoCancellation is a "master switch" for disabling all audio processing.  set to disableAP=true and stereo=true allows fullband, stereo sound.
AGC: misbehaves with certain 'pre-gained' mics.
HPF: can't hear any effect but it's the last constraint to make configurable.

* Update RTCUtils.js

Avoids having to do browser checks twice.

Fixes: #423

Logic broken while refactoring (in b8a2a7baaf [eslint] no-unneeded-ternary).