Split app move into separate copy/delete steps.

App movement now has three distinct stages: copying, scanning, and
cleanup.  Previously, a battery pull late in the move process would
end up with packages.xml pointing at the old location which had been
torn down.  Now, we update packages.xml to point at the new location
as the "source of truth" before we start deleting the old location.

Bug: 21831336
Change-Id: I6f57f37a8cb335127db9ebb7c6b6cfe5755ada99