Enable cmdline nandroid backup of vold volumes

Issuing 'adb shell nandroid backup' just after recovery has started on a
device with vold managed primary storage will fail because vold has not
initd for nandroid. Let nandroid call vold_init. Subsequent calls to
vold_init should be of no concern since sock will be set and
vold_client_start() will return immediately.

Change-Id: I88b16ff0a8c5b058b58f81095f63374bfbf70b34