vold_prepare_subdirs sets policy in vold-created dirs.

Bug: 25861755
Test: Boot device, observe logs
Change-Id: I6c13430d42e9794003eb48e6ca219b874112b900
Merged-In: I6c13430d42e9794003eb48e6ca219b874112b900
(cherry picked from commit 47f3ed09d222ee126cf2fe23b5fe15cd0b64520e)
diff --git a/private/vold_prepare_subdirs.te b/private/vold_prepare_subdirs.te
new file mode 100644
index 0000000..c2146f9
--- /dev/null
+++ b/private/vold_prepare_subdirs.te
@@ -0,0 +1 @@
+domain_auto_trans(vold, vold_prepare_subdirs_exec, vold_prepare_subdirs)