Add an option to disable including `libavcodec.so` in the firefox container.

Right now, the firefox container explcitly includes the "best" libavcodec shared library (dlopen()ed by firefox), 100% of the time as a concession to usability.

Since this is optional, and there are usage models that do not require video/audio playback, this should be made optional like how PulseAudio currently is, under the rationale that people who can live with omitting it, can gain reduced attack surface.