We should avoid creating more than one session connection or more than
one system connection. We should also ideally avoid blocking the main
thread.
For now this still uses blocking in a couple places, but wrapping async
code (which is how `zbus::blocking` is implemented anyway).
This moves the `NameOwners` creation out of `A11yKeyboardMonitorState`,
so it can be shared with other things. We will likely want that for
https://github.com/pop-os/cosmic-comp/pull/465 to define a secured
protocol to pass an fd to the portal, and potentially any other
sensitive DBus protocols implemented by the compositor.
