Allow update_engine to get gsid property

Allow the update_engine to use the gsid property and to avoid the VAB
merge when running a DSU.

Bug:147071959
Test: ota_e2etest.py

Change-Id: I40220877625453198b217e788e6b3bfab8437f24
diff --git a/private/update_engine.te b/private/update_engine.te
index a76ab49..539399e 100644
--- a/private/update_engine.te
+++ b/private/update_engine.te
@@ -11,3 +11,6 @@
 
 # Allow to set the OTA related properties, e.g. ota.warm_reset.
 set_prop(update_engine, ota_prop)
+
+# Allow to get the DSU status
+get_prop(update_engine, gsid_prop)