Limit rename() workaround to /storage/emulated.

This workaround was intended to silently fix EXDEV move failures due to
the /Android/data and /Android/obb bind mounts. However, the workaround
should be limited to moves to *and* from the emulated filesystem. For
moves from the emulated filesystem to another filesystem (or
vice-versa), this would never have worked in the first place, and we
want to give the app this feedback, so it knows it needs to do a more
expensive copy operation and can show this in the UI. We know some apps
(like DocsUI) already handle this.

Public volumes (eg /storage/ABCD-1234) don't need this workaround, since
they don't have the bind mounts. Private volumes that aren't primary
don't have these bind mounts either.

Bug: 146430607
Test: N/A
Change-Id: I7bfe88e07708fe044ce8df02000a97cfad19bdee