util: add utiliy functions for system settings and runtime info

util: add utiliy functions for system settings and runtime info

System settings and runtime info API allows to register only one global
callback per key. In case when multiple indicator modules wants to listen
on the same key changes API enforces some cross module dependencies.

Adding utility function for handling multiple callbacks per key solves this
issue, making module more independent of each other.

Change-Id: Ie760ab7184a6a7f5ca402e6ef1db2350b57a2de2
Signed-off-by: Lukasz Stanislawski <l.stanislaws@samsung.com>