session-helper: Return G_DBUS_METHOD_INVOCATION_HANDLED where appropriate
session-helper: Return G_DBUS_METHOD_INVOCATION_HANDLED where appropriate

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak