samsung: hidl: add fastcharge implementation

samsung: hidl: add fastcharge implementation

 * On samsung, kernel exposes a sysfs node to disable fastcharge 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 the fastcharge HIDL, which writes in the node

              /sys/class/sec/switch/afc_disable

   0 or 1 depeding on user selection.
   0 means that fastcharge is enabled, while 1 means that fastcharge is
     disabled

Change-Id: I369ba9c437b3a83a88c2ce74d603b7d7ddd9cfbb