Do not allow system_server to access SDcard files.

As per:
https://android-review.googlesource.com/#/c/84130/3/system_server.te@240


it is unsafe to allow such access.

Add a neverallow rule to prohibit any rules on sdcard_type in the
future.

Change-Id: Ife714b65b07144eb6228a048a55ba82181595213
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>