power: refactor power operations

power: refactor power operations

 Fix wake lock/unlock operation to have prefix of "power". And it
replaces indirect invocation through device_notify(), which is often
in iot-headless.
 Add functions for add/remove poweroff wait for internal usage. It
is similar to the existing add/remove dbus methods, but only differs
in that the function can be invoked by the deviced itself.

Change-Id: Ide3ceb5f894b376901637992d66401a110eafbf0
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>