Allow vold to change priority when benchmarking.

avc: denied { sys_nice } for capability=23 scontext=u:r:vold:s0 tcontext=u:r:vold:s0 tclass=capability permissive=0

Bug: 21711477
Change-Id: I78e7a6667e06a4b1a2b0c4d26ddae4797231e553

vold.te

@@ -156,6 +156,9 @@ allow vold vold_data_file:file create_file_perms;
 allow vold init:key { write search setattr };
 allow vold vold:key { write search setattr };
 
+# vold temporarily changes its priority when running benchmarks
+allow vold self:capability sys_nice;
+
 neverallow { domain -vold } vold_data_file:dir ~{ open create read getattr setattr search relabelto ioctl };
 neverallow { domain -vold } vold_data_file:notdevfile_class_set ~{ relabelto getattr };
 neverallow { domain -vold -init } vold_data_file:dir *;