MUApplicationDelegate: use Opus (and fall back to CELT) for the 'Low' quality mode.

Murmur 1.2.4 enforces that clients use Opus when in Opus mode. This means that the
iOS client's Speex packets will be silently discarded by Murmur.
(cherry picked from commit 0b62a84f91a88e2fc90ffbaa5bb4df4c84b7a276)