Propagate NOTIFY_SOCKET down to droid-init-done.sh

NOTIFY_SOCKET has changed from @/org/freedesktop/systemd1/notify to
/run/systemd/notify between systemd 208 and 216, so a hard-coded value
breaks droid-hal-init with certain systemd version that wants
/run/systemd/notify.

The clean way is to propagate NOTIFY_SOCKET from droid-hal-startup.sh
down to droid-init-done.sh.