[mte] add property to globally enable mte.

Bug: 216305376

Change-Id: I25d0b3c9d0e7e6bba14eedf9b833c5e07786ec71
diff --git a/private/property_contexts b/private/property_contexts
index 5a9f0e8..1f6ba3f 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -668,6 +668,7 @@
 
 # shell-only props for ARM memory tagging (MTE).
 arm64.memtag. u:object_r:arm64_memtag_prop:s0 prefix string
+persist.arm64.memtag.mode u:object_r:arm64_memtag_prop:s0 exact string
 
 net.redirect_socket_calls.hooked u:object_r:socket_hook_prop:s0 exact bool