fingerprint: autostart fprintd when necessary

commit e333263f changed fingerprint.js
to not throw an exception when fprintd is uninstalled, by adding the
flags DO_NOT_LOAD_PROPERTIES and DO_NOT_AUTO_START

DO_NOT_LOAD_PROPERTIES is correct.  Loading the properties is what makes
it fail at initialization time when not installed.  DO_NOT_AUTO_START is
not correct though. It means fprintd will never get activated implicitly
when we need it.

This commit removes DO_NOT_AUTO_START thus making fprintd start when we
need it, but not fail at initialization time when not around.

https://bugzilla.gnome.org/show_bug.cgi?id=683131