audio: stash active inputs in videomanager

Needed for audio streaming because the audio input needs to be accessed
at call level so it can tell the input to switch to a file or a
different input.

Getting a shared pointer to an AudioInput should only be done during
setup, so as to not walk the whole map in audio processing loops.

Change-Id: I49be1cb3c641b50e6f70356f330d40e1c27bef61