Delay vold connectors until published, fix NPE.

If they connect too quickly, PackageManager could end up trying to
obtain the yet-unpublished MountService.

Fix NPE in UserManagerService when trying to persist fingerprints,
and fix write ordering to always write [id].xml before userlist.xml
to avoid battery pull issues.  Simlarly, delete [id].xml only after
updating userlist.xml.

Bug: 27869443
Change-Id: I43d8552e5e37b9ca4137cca3e3e76684c7dee605