sdcard: mount sdcard with the noexec option

Vold mounts the sdcard with noexec, but the fuse deamon
mounts with exec, so it is still possible to execute
binaries:

  /dev/fuse /storage/sdcard1 fuse rw,nosuid,nodev,relatime,
  user_id=1023,group_id=1023,default_permissions,allow_other 0 0

  /dev/block/vold/179:65 /mnt/media_rw/sdcard1 vfat rw,dirsync,
  nosuid,nodev,noexec,relatime,uid=1023,gid=1023,fmask=0007,
  dmask=0007,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,
  shortname=mixed,utf8,errors=remount-ro 0 0

With this change both vold and fuse mounts with noexec.

Change-Id: I66cbfc3a3a89a26958f83577f5e7a5e27f99184e