main: Do not export DBus interfaces before initializing the UI

main: Do not export DBus interfaces before initializing the UI

Since commit 7cdb75e7ce76, initializing UI is deferred until the session
mode has been loaded. However DBus is still initialized immediately,
which means that for DBus methods that access properties in Main, there
is now a window between the method being exposed on the bus and the
method being ready to be called. At least g-s-d grabbing global keybindings
is likely to fall in this window on session startup, and almost guaranteed
when regrabbing bindings after a shell restart.
To fix, defer initializing DBus as well.

https://bugzilla.gnome.org/show_bug.cgi?id=694321