Invoke config changed callback directly

Invoke config changed callback directly

- Issue:
If the app changes the config using setting API, the framework did not
invoke config changed callbacks.

- Solution:
In previous code, vc_config_mgr module could know the changes of the
configuration through checking the difference between the values in the
memory and actual configuration file. However, the values in the memory
and configuration file are the same if the app uses the setting API.
This is beecause the setting API sets the value in both the memory and
file.
So, this patch adds a new logic for invoking the callbacks. Through this
patch, vc_config_mgr module will directly invoke the callbacks related
to changed values when the app changes the configuration using setting
API.

Change-Id: Ic8023395889021501d8528401f9058b743699c72
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>