sepolicy: allow rules for apk verify system property

ro.apk_verity.mode was introduced in P on crosshatch. This change
changes the label from default_prop to a new property, apk_verity_prop.

ro.apk_verity.mode is set by vendor_init per build.prop, in order to
honor Treble split.  It is also read by system_server and installd
currently.

Test: verify functioning without denials in dmesg
Bug: 142494008
Bug: 144164497
Change-Id: I1f24513d79237091cf30025bb7ca63282e23c739
diff --git a/private/system_server.te b/private/system_server.te
index 603ea9c..7ddaf4a 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -635,6 +635,9 @@
 # Read the property that mocks an OTA
 get_prop(system_server, mock_ota_prop)
 
+# Read the property as feature flag for protecting apks with fs-verity.
+get_prop(system_server, apk_verity_prop)
+
 # Create a socket for connections from debuggerd.
 allow system_server system_ndebug_socket:sock_file create_file_perms;