Allow update_engine to read virtual ab feature flag.

Test: adb shell getprop -Z [the two flags]
Bug: 135752105

Change-Id: I3e0372e969ea0161787c32560a8c40fed2953619
diff --git a/private/property_contexts b/private/property_contexts
index 254c55a..d1a97d9 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -212,3 +212,7 @@
 
 # Charger properties
 ro.charger.             u:object_r:charger_prop:s0
+
+# Virtual A/B properties
+ro.virtual_ab.enabled   u:object_r:virtual_ab_prop:s0
+ro.virtual_ab.retrofit  u:object_r:virtual_ab_prop:s0