Show Battery saver tip when battery saver is on

Before this CL, we only show "Battery Saver is on" when users turn it
on in battery tips.

In this CL, we change to constantly show this tip when battery
saver is on.

Change-Id: I9ddefdbd5818725202ddf4469b4a685ba008060a
Fixes: 75251000
Test: RunSettingsRoboTests