Allow system_server to read oat dir

Required for PackageManagerService to perform restorecon recursively on a
staging dir.

Addresses the following denial:
avc: denied { open } for name="oat" dev="mmcblk0p28" ino=163027 scontext=u:r:system_server:s0 tcontext=u:object_r:dalvikcache_data_file:s0 tclass=dir

Bug: 19550105
Bug: 20087446
Change-Id: I0f6ebb79745091ecb4d6d3dbe92f65606b7469da

system_server.te

@@ -14,6 +14,7 @@ allow system_server system_server_tmpfs:file execute;
 
 # For art.
 allow system_server dalvikcache_data_file:file execute;
+allow system_server dalvikcache_data_file:dir r_dir_perms;
 
 # /data/resource-cache
 allow system_server resourcecache_data_file:file r_file_perms;