Show notification to promote window magnification mode available

The magnification mode is full-screen when the migrating deivce turns on
magnification settings.
We show a notification prompt the user that new window magnifier to
guide the user to take a look.

Bug: 168635084
Test: 1. adb shell settings put secure \
      accessibility_show_window_magnification_prompt 1
      2. use full-screen magnification to see if it works well.
      atest WindowMagnificationPromptControllerTest
      atest FullScreenMagnificationGestureHandlerTest
Change-Id: I40dfc49307d2dad380f1e662960d35f5cc5e7524
diff --git a/proto/src/system_messages.proto b/proto/src/system_messages.proto
index 15bd4dc..ae024ff 100644
--- a/proto/src/system_messages.proto
+++ b/proto/src/system_messages.proto
@@ -332,5 +332,9 @@
     // Notify the user that the admin suspended personal apps on the device.
     // Package: android
     NOTE_PERSONAL_APPS_SUSPENDED = 1003;
+
+    // Notify the user that window magnification is available.
+    // package: android
+    NOTE_A11Y_WINDOW_MAGNIFICATION_FEATURE = 1004;
   }
 }