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  In order to protect the '/android_secure' directory on VFAT removable media
from being mucked with by 3rd party applications on the device, we hide the
directory with a read-only, zero-sized tmpfs mounted on-top. A reference to the
hidden directory is kept by a bind-mount which is mounted at a location which
only root can access.

Staging consists of:
  1. Mount checked media at a secure location (/mnt/secure/staging)
  2. Ensure /android_secure exists on the media, (creating if it doesnt)
  3. Bind-mount /mnt/secure/staging/android_secure -> /mnt/secure/asec
     (where only root can access it)
  4. Mount an RDONLY zero-sized tmpfs over /mnt/secure/staging/android_secure
  5. Atomically move /mnt/secure/staging to the publicly accessable storage
     directory (/mnt/sdcard)
Signed-off-by: San Mehat <san@google.com>