Migrate primary external storage.

Wire up through MountService to call down into vold.  Watch for
unsolicited events that report progress, including special value "82"
that signals that copy has finished.  We use this value to persist
the volumeUuid in case of unexpected reboot, since it indicates the
new volume is ready.

Wire progress updates through existing callback pipeline.

Update the volume mounting code to match against the persisted UUID
when selecting the primary external storage.

Bug: 19993667
Change-Id: Id46957610fb43517bbfbc368f29b7d430664590d