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We really only support a single emulated volume on the device at a
time, either on internal storage, or moved to a private volume.  To
avoid kicking off a giant rescan of all media when moved, keep all
the paths the same when mounted as primary.

Also ensure we have /data/media/0 ready on private volumes.

Bug: 20275423
Change-Id: I0c102f430b865ca7536772b1fae56d8c9660a97a