Adds system_user_mode_emulation_prop property.

It will be used by system_server only (i.e., not even Shell) to let
developers change the system user mode (to be headless or full).

Test: sesearch --allow -t system_user_mode_emulation_prop $ANDROID_PRODUCT_OUT/vendor/etc/selinux/precompiled_sepolicy

Bug: 226643927

Change-Id: Iaba42fd56dce0d8d794ef129634df78f9599260f
diff --git a/private/property_contexts b/private/property_contexts
index 1ad19c1..2fafdf5 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -57,6 +57,7 @@
 persist.nfc.            u:object_r:nfc_prop:s0
 persist.nfc_cfg.        u:object_r:nfc_prop:s0
 persist.debug.          u:object_r:persist_debug_prop:s0
+persist.debug.user_mode_emulation u:object_r:system_user_mode_emulation_prop:s0
 logd.                   u:object_r:logd_prop:s0
 persist.logd.           u:object_r:logd_prop:s0
 ro.logd.                u:object_r:logd_prop:s0