Add mmd.enabled_aconfig to sepolicy

"mmd --set-property" command copies AConfig flag to
"mmd.enabled_aconfig" system property to control mmd whether to start
or not at aosp/3368262.

Bug: 375431994
Test: manual

Change-Id: I27691243d9eed73fff9baccd9d23a346c18d7f4c
diff --git a/private/property_contexts b/private/property_contexts
index 163c873..eab1e86 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -1776,3 +1776,6 @@
 # Properties that allows vendors to enable Trusty security VM features
 trusty.security_vm.enabled u:object_r:trusty_security_vm_sys_vendor_prop:s0 exact bool
 trusty.security_vm.keymint.enabled u:object_r:trusty_security_vm_sys_vendor_prop:s0 exact bool
+
+# Properties for mmd
+mmd.enabled_aconfig u:object_r:mmd_prop:s0 exact bool