Allow vold to set MLS level on per-user directories.

We want to extend vold_prepare_subdirs to set the MLS level to the
correct per-user value for selected user-specific directories.

Grant vold_prepare_subdirs the access it needs to do this, and allow
vold to access the temporary property controlling this.

Bug: 141677108
Test: Manual, with and without property set.
Change-Id: I572462cfd9b8869381f2af5faa29165bb8373d4b
diff --git a/private/property_contexts b/private/property_contexts
index 1fe3e0c..4f7a1dc 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -115,6 +115,9 @@
 vold.                   u:object_r:vold_prop:s0
 ro.crypto.              u:object_r:vold_prop:s0
 
+# TODO(b/141677108): Remove once true everywhere
+ro.vold.level_from_user          u:object_r:vold_config_prop:s0
+
 # ro.build.fingerprint is either set in /system/build.prop, or is
 # set at runtime by system_server.
 ro.build.fingerprint    u:object_r:fingerprint_prop:s0 exact string