Settings: Add FastCharge preference into Battery settings

 * Several OEMs let the user decide whether to enable or disable quick
   charging technology when using a quickcharge charger.
   Samsung, for example, exposes a sysfs node to disable it at
   will, depending on what the user sets in battery settings UI.

 * Disabling fast charge may be useful for reducing the heat produced by
   the device while charging, or for extending the lifespan of the battery.

 * This commit introduces a switch preference for disabling fastcharge
   on devices that support said feature.

Change-Id: I7dd09d357e9bd555a8efeaf9ee191e52b9f2d151