Extreme battery saver: Show warning notification instead of toast

(Just for now; we'll remove it before shipping.)

Bug: 68769804
Test: manual test
Change-Id: Ib1d58fbc88f79a62030a2e59d1dce15e9ff1323a
diff --git a/proto/src/system_messages.proto b/proto/src/system_messages.proto
index 52f721b..75d080d 100644
--- a/proto/src/system_messages.proto
+++ b/proto/src/system_messages.proto
@@ -223,6 +223,9 @@
     // Package: com.android.systemui
     NOTE_TV_PIP = 1100;
 
+    // Extreme battery saver notifiaction.
+    NOTE_BATTERY_SAVER_WARNING = 1200;
+
     // Notify the user that open Wi-Fi networks are available.
     // Package: android
     NOTE_NETWORK_AVAILABLE = 17303299;