The implementations have been imported into SettingsLib. Setting's copy
can now be removed, which this change also does.
Test: Manually check battery status, which uses FooterMixin, looks OK.
make RunSettingsLibRobotTests && make RunSettingsRoboTests
&& make RunSettingsGoogleRoboTests
Change-Id: I6539605fdad80d156ff5ff249e68df4a1c412067