configstore-utils: add feature to cache service handles

Previously, the service handle for the configstore needs to be retrieved
for every calls. This change reduces the time for retrieving multiple
configstore items from a same client by caching the service handle per a
configstore interface class.

Bug: 38411900
Test: built and tested on sailfish.
Change-Id: Iaff81e9f2b026aee0bd3865f5d5ed3fb7b6056e0
1 file changed