This commit essentially moves the get_peer_credentials_async() function
from MwsScheduleService into a new MwsPeerManagerDBus class, which has
its interface abstracted as MwsPeerManagerInterface. The interface
abstraction will be used in future to simplify unit testing of the
scheduler.

The scheduler now has a MwsPeerManager, which it uses to get the
credentials of peers so that it can prioritise schedule entries from
some peers (like eos-updater or gnome-software) over others. It does
this by matching the peer’s executable path. Using an LSM label from the
kernel would be better (it would avoid some race conditions with looking
up PIDs, and avoid the possibility of /proc/$pid/exe being forged by
processes) — but since we don’t use an LSM in EOS, we can’t use any LSM
labels.

Signalling of peers vanishing has moved from MwsScheduleService into the
MwsPeerManager too, which means the MwsScheduler can now be notified
directly of peers vanishing, rather than the MwsScheduleService having
to propagate a notification to it with
mws_scheduler_remove_entries_for_owner().

The overall aim of this commit (which it achieves) is to allow
prioritising schedule entries by:
 1. App extensions to com.endlessm.* apps (most important)
 2. OS updates
 3. App updates and app extensions to non-Endless apps
 4. Any other schedule entries (least important)

This is achieved by prioritising schedule entries by the peer which adds
them, and then prioritising within that by the priority provided by the
peer. Schedule entries from eos-updater and gnome-software are handled
specially: they are ranked equally as peers, so the priorities they set on
schedule entries are scoped together. By setting high and low priorities
in gnome-software for different kinds of app updates, and medium
priorities in eos-updater for OS updates, we should get rankings 1–3
from above. Ranking 4 is given by prioritising schedule entries from
peers other than gnome-software and eos-updater below them.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://phabricator.endlessm.com/T21327