Add auto battery saver suggestion notification/dialog

Spec: go/ebs-low-battery-mode-flow

- Basically when the user manually enables battery saver 4 times,
we show this notification to suggest turning on "scheduled"
(i.e. auto) battery saver.

- We show it through 8th time. If the user hits "no thanks",
or if auto-saver is enabled already, we will not show it.

- Introduced a new notification channel "HINTS" with
IMPORTANCE_DEFAULT.

Bug: 74120126
Test: Manual test with ll development/scripts/battery_simulator.py

Change-Id: I713abc59dc7caee6882ba848c3e3aabaf778c2bd
diff --git a/proto/src/system_messages.proto b/proto/src/system_messages.proto
index 608970f..f9af31c 100644
--- a/proto/src/system_messages.proto
+++ b/proto/src/system_messages.proto
@@ -208,6 +208,10 @@
     // Package: android
     NOTE_ZEN_UPGRADE = 48;
 
+    // Notification to suggest automatic battery saver.
+    // Package: android
+    NOTE_AUTO_SAVER_SUGGESTION = 49;
+
     // 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