sdcardd: grant unmount

If the sdcard daemon is restarted (crash or otherwise), one of the first
things it attempts to do is umount the previously mounted /mnt/shell/emulated
fuse filesystem, which is denied by SELinux with the following denial:

  sdcard  : type=1400 audit(0.0:6997): avc: denied { unmount } for scontext=u:r:sdcardd:s0 tcontext=u:object_r:fuse:s0 tclass=filesystem permissive=0

Allow the operation.

Steps to reproduce:

  1) adb shell into the device and su to root
  2) run "kill -9 [PID OF SDCARD]

Expected:

  sdcard daemon successfully restarts without error message.

Actual:

  SELinux denial above, plus attempts to mount a new filesystem
  on top of the existing filesystem.

(cherrypicked from commit abfd427a)

Bug: 17383009
Change-Id: I386bfc98e2b5b32b1d11408f7cfbd6e3c1af68f4