battery: introduce battery-plugin for iot-headless

battery: introduce battery-plugin for iot-headless

It handles battery event according to the iot-headless policy. Those
policies are specified in battery.conf. For now it only handles event
about charger connect/disconnect.
 1. It triggers power state transition. Those transition actions are
    specified in EVENT_ACTION section.
 2. It might hold wakelock for charger connection. It is specified in
    CHARGER_WAKELOCK section.

Change-Id: Iee6803604a25bacca5ce68953033b71560ed4ba8
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>