Bug#805414: gdm3: disable pulseaudio to prevent capturing A2DP sink on session start

[Matthew Gabeler-Lee]

Package: gdm3
Version: 3.20.1-1
Followup-For: Bug #805414

It seems like gdm3 should not be claiming the A2DP audio interface, or
really _any_ audio interface when it is not 'active' / visible.

It needs the keyboard to accept passwords, but it doesn't prevent me from
using my keyboard after I log in ;)

I realize the PA bits are going to be (much) more complicated than the
keyboard example, but the principle applies I think.

Being ignorant of PA, I'm also a bit confused as to why A2DP is special
here.  Other sound devices aren't broken by gdm3 wanting to have sound
services available for accessibility, why is A2DP different here?