configstore: use utility functions defined in ConfigStoreUtils.h

This change modifies existing implementation to get config values via
configstore by using utility functions defined in ConfigStoreUtils.h.

Specifically, as an example, the utility function getInt64() is used
for vsyncEventPhaseoffsetNs defined in ISurfaceFlingerConfigs.

Design doc: go/design-confighal
Bug: 34724435
Test: build, run

Change-Id: I82a9371a84902c1e185d619d77e4c2e3272f84db