Add gki.prevent_downgrade_*

Add ro.build.ab_update.gki.prevent_downgrade_{version,spl} for
update_engine to determine whether downgrade in kernel version or SPL is
considered an error or not.

Bug: 162623577
Test: update_engine_unittest
Test: apply OTA

Change-Id: If602924d50a2d5cfb3c256b82491c413a9d39f9d
diff --git a/private/compat/30.0/30.0.ignore.cil b/private/compat/30.0/30.0.ignore.cil
index 690350c..a63a50e 100644
--- a/private/compat/30.0/30.0.ignore.cil
+++ b/private/compat/30.0/30.0.ignore.cil
@@ -5,6 +5,7 @@
 (typeattribute new_objects)
 (typeattributeset new_objects
   ( new_objects
+    ab_update_gki_prop
     adbd_config_prop
     apex_info_file
     debugfs_kprobes
diff --git a/private/property_contexts b/private/property_contexts
index 3d02e97..b9f252b 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -703,7 +703,11 @@
 
 ro.boringcrypto.hwrand u:object_r:exported_default_prop:s0 exact bool
 
-ro.build.ab_update         u:object_r:exported_default_prop:s0 exact string
+# Update related props
+ro.build.ab_update                                u:object_r:exported_default_prop:s0 exact string
+ro.build.ab_update.gki.prevent_downgrade_version  u:object_r:ab_update_gki_prop:s0 exact bool
+ro.build.ab_update.gki.prevent_downgrade_spl      u:object_r:ab_update_gki_prop:s0 exact bool
+
 ro.build.expect.baseband   u:object_r:exported_default_prop:s0 exact string
 ro.build.expect.bootloader u:object_r:exported_default_prop:s0 exact string