Activate night mode when battery saver

This as originally in SysUI but UiModeManagerService is cleaner way
of doing it. This way SysUI won't have to keep track of user preferences
or override user settings.

Test: manually toggle battery saver and observe QS
Bug: 104205224
Change-Id: Ic0d24e36eec2ad3b66b32960bfc5e659470044b1