Allow update_engine, recovery, and fastbootd to read snapuserd properties.

Bug: 193833730
Test: OTA applies and boots
Change-Id: I81c089e1763a7e25b23df245f76e04acd52a337e
diff --git a/private/update_engine.te b/private/update_engine.te
index d828e1f..c3f575f 100644
--- a/private/update_engine.te
+++ b/private/update_engine.te
@@ -24,6 +24,7 @@
 # Allow to communicate with the snapuserd service, for dm-user snapshots.
 allow update_engine snapuserd:unix_stream_socket connectto;
 allow update_engine snapuserd_socket:sock_file write;
+get_prop(update_engine, snapuserd_prop)
 
 # Allow to communicate with apexd for calculating and reserving space for
 # capex decompression