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