system_server: Allow hwservicemanager to make binder calls

Fixes the following denial:
avc: denied { call } for pid=791 comm="system_server" scontext=u:r:system_server:s0 tcontext=u:r:hwservicemanager:s0 tclass=binder permissive=1

Test: Builds, boots, vibrator works on bullhead
Change-Id: I56a0a86b64f5d46dc490f6f3255009c40e6e3f8f

public/system_server.te

@@ -555,6 +555,9 @@ r_dir_file(system_server, proc_net)
 r_dir_file(system_server, rootfs)
 r_dir_file(system_server, sysfs_type)
 
+# Allow system_server to make binder calls to hwservicemanager
+binder_call(system_server, hwservicemanager)
+
 
 ###
 ### Neverallow rules