Move to downloads root instead of home root.

Previously we assume the home root exists always, but while migrating
internal storage, the home root is temporary removed from Android. Since
we open donwnloads root by default now, the CL lets Files app move to
the downloads directory instead of home root when the current directory
is removed.

BUG=27570929

Change-Id: I89efb1ae32ba8ae7269b3242035d5b67114fcb54