volume: Show indicator when microphone is active

Devices like cameras and microphones are privacy sensitive, as they can
be used to spy on the user. We cannot prevent non-sandboxed apps from
doing that, but as we already track when the microphone is recording,
we can at least show an indicator to make sure it doesn't happen behind
the user's back.

GNOME/gnome-shell!729
6 jobs for !729 with add-microphone-indicator in 10 minutes and 16 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #469041
check_commit_log

00:00:28

passed #469043
eslint

00:01:09

passed #469042
js_check

00:00:28

 
  Build
passed #469044
build

00:08:07

 
  Test
passed #469045
test

00:00:59

passed #469046
test-pot

00:00:56