Settings: Battery saver settings sub-page.

New battery saver settings page, accessed via
the power usage settings overflow menu.

The settings page has a master switch to toggle
battery saver mode, feature disclosure text,
and a preference to configure the automatic
trigger level.

Remove developer checkbox now that this option
has a real home.

NO_SQ: multi project change
Bug:13329308
Change-Id: Iac54238f3406439711b44a3c17f220ac5e370a37