Set apex. property as "system_restricted"

Since the property is supposed to be used by vendor-side .rc file as
read-only (especially by vendor apex), it should be "system_restricted".

Also allow vendor_init to read the property.

Bug: 232172382
Test: boot cuttlefish (with vendor apex using the property)
Change-Id: I502388e550e0a3c961a51af2e2cf11335a45b992
diff --git a/private/vendor_init.te b/private/vendor_init.te
index 70b3ef9..acbd84e 100644
--- a/private/vendor_init.te
+++ b/private/vendor_init.te
@@ -12,6 +12,9 @@
 # Let vendor_init react to AVF device config changes
 get_prop(vendor_init, device_config_virtualization_framework_native_prop)
 
+# Let vendor_init use apex.<name>.ready to start services from vendor APEX
+get_prop(vendor_init, apex_ready_prop)
+
 # chown/chmod on devices, e.g. /dev/ttyHS0
 allow vendor_init {
   dev_type