Give secondary users read-only physical cards.

Long ago, we mounted secondary physical cards as readable by all
users on the device, which enabled the use-case of loading media on
a card and viewing it from all users.

More recently, we started giving write access to these secondary
physical cards, but this created a one-directional channel for
communication across user boundaries; something that CDD disallows.

This change is designed to give us the best of both worlds: the
package-specific directories are writable for the user that mounted
the card, but access to those "Android" directories are blocked for
all other users.  Other users remain able to read content elsewhere
on the card.

Bug: 22787184
Change-Id: Ied8c98995fec1b7b50ff7d930550feabb4398582