Add UUID (re-)scan support for vold-mounted volume (2/2)

Consider the following scenario:
* Using physical primary only storage cofigurations with the sdcard
  daemon
* On first boot, the UUID is plumbed to the framework via vold, but this
  is only done when a volume is mounted (via the extractMetadata()
  function).
* When the framework crashes/restarts, vold does not need to mount any
  volumes (as the sdcard daemon will still be running), which means
  that the UUID is never plumbed to the framework upon restart.
* MediaProvider will continuously crash with an NPE due to the UUID being
  null.

Workaround the issue by adding "scanuuid" to the "vdc volume" command
list, and trigger a scan when the framework detects that a volume is
mounted but the UUID is null.

Change-Id: I4a48a1612ea7d6fcba7150bfc66c03045a86e253