UnitTestFrameworkPkg: Modify APIs in UnitTestPersistenceLib

UnitTestFrameworkPkg: Modify APIs in UnitTestPersistenceLib

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4183

UnitTestPersistenceLib now consumes private struct definition.
Modify APIs in UnitTestPersistenceLib to make it easy to become
a public library.

Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>