Grant gmscore permission to read virtual ab properties

Bug: 168059475
Test: Serve an update over gmscore
Change-Id: Iefd88f4189b50ee68ee09bcb5a20556ba4ea3e1a
diff --git a/private/property_contexts b/private/property_contexts
index 4785707..d65d1d6 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -235,8 +235,8 @@
 ro.enable_boot_charger_mode u:object_r:charger_config_prop:s0 exact bool
 
 # 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
+ro.virtual_ab.enabled   u:object_r:virtual_ab_prop:s0 exact bool
+ro.virtual_ab.retrofit  u:object_r:virtual_ab_prop:s0 exact bool
 
 ro.product.ab_ota_partitions u:object_r:ota_prop:s0 exact string
 # Property to set/clear the warm reset flag after an OTA update.