Add notification for Test Harness Mode

As part of Security Review's requirements, I need to add a notification to
Test Harness Mode. It is visible iff Test Harness Mode is active (unless
the user blocks notifications from the system).

Currently, tapping on it does nothing; there's no Intent available in
the system to direct the user to perform a factory reset.

Bug: 80137798
Test: make && adb shell cmd testharness enable; verify that the dialog
exists
Change-Id: I9b391bce57ec1ef5392bd78bfdcf62a3a7e00e55
diff --git a/proto/src/system_messages.proto b/proto/src/system_messages.proto
index 3a89316..2f34ce6 100644
--- a/proto/src/system_messages.proto
+++ b/proto/src/system_messages.proto
@@ -226,6 +226,10 @@
     // Inform that user that the USB port is free of contaminants.
     NOTE_USB_CONTAMINANT_NOT_DETECTED = 53;
 
+    // Inform the user that Test Harness Mode is active.
+    // Package: android
+    NOTE_TEST_HARNESS_MODE_ENABLED = 54;
+
     // ADD_NEW_IDS_ABOVE_THIS_LINE
     // Legacy IDs with arbitrary values appear below
     // Legacy IDs existed as stable non-conflicting constants prior to the O release