allow systemserver to sigkill clat

This appears to be an oversight in T sepolicy???

Based on observed logs (on a slightly hacked up setup):

04-04 20:38:38.205  1548  1935 I Nat464Xlat: Stopping clatd on wlan0
04-04 20:38:38.205  1548  1935 I ClatCoordinator: Stopping clatd pid=7300 on wlan0
04-04 20:38:43.408  1548  1548 W ConnectivitySer: type=1400 audit(0.0:8): avc: denied { sigkill } for scontext=u:r:system_server:s0 tcontext=u:r:clatd:s0 tclass=process permissive=0
04-04 20:38:43.412  1548  1935 E jniClatCoordinator: Failed to SIGTERM clatd pid=7300, try SIGKILL
04-04 20:39:27.817  7300  7300 I clatd   : Shutting down clat on wlan0
04-04 20:39:27.819  7300  7300 I clatd   : Clatd on wlan0 already received SIGTERM
04-04 20:39:27.830  2218  2894 D IpClient/wlan0: clatInterfaceRemoved: v4-wlan0
04-04 20:39:27.857  1548  1935 D jniClatCoordinator: clatd process 7300 terminated status=0

I think this means SIGTERM failed to work in time, and we tried SIGKILL and that was denied, and then the SIGTERM succeeded?

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ia55ebd812cb9e7062e3cb10d6cb6851638926868
diff --git a/private/system_server.te b/private/system_server.te
index 20e6427..ff863cd 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -1185,7 +1185,7 @@
 
 # Allow system_server to start clatd in its own domain and kill it.
 domain_auto_trans(system_server, clatd_exec, clatd)
-allow system_server clatd:process signal;
+allow system_server clatd:process { sigkill signal };
 
 # ART Profiles.
 # Allow system_server to open profile snapshots for read.