Fix dark theme and battery saver interaction

In settings we weren't properly taking into account battery saver
enabled dark theme. This CL makes it so that when battery saver
on we disable the toggle until battery saver is off since
you can't change dark theme in battery saver. Additionally it
adds a message indicating why the toggle was disabled so the
user doesn't get confused.

Test: robotests
Bug: 131913864
Change-Id: I855e429a666098c837650852d1e1355477afa13d