Allow system watchdog to collect traces from vold.

We're investigating a bug where vold gets wedged, and we need to
collect ANR stack traces from it to debug further.

avc: denied { signal } for comm="watchdog" scontext=u:r:system_server:s0 tcontext=u:r:vold:s0 tclass=process permissive=0
avc: denied { ptrace } for scontext=u:r:crash_dump:s0 tcontext=u:r:vold:s0 tclass=process permissive=0

Bug: 122090837
Test: manual
Change-Id: I738e63717715189b9ae2317472f671e3563afaa9
diff --git a/private/system_server.te b/private/system_server.te
index 2cf5ea7..27407f0 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -276,6 +276,7 @@
   sdcardd
   statsd
   surfaceflinger
+  vold
 
   # This list comes from HAL_INTERFACES_OF_INTEREST in
   # frameworks/base/services/core/java/com/android/server/Watchdog.java.