libsnapshot: Add GetSnapshotMergeStatsInstance

This is preferred over SnapshotMergeStats::GetInstance because
the latter needs a concrete SnapshotManager, not the ISnapshotManager
interface.

SnapshotManagerStub::GetSnapshotMergeStatsInstance returns
a SnapshotMergeStatsStub instance.

Test: vts_libsnapshot_test
Bug: 148956645
Change-Id: Ife0ad6d3ce85333cbf395d07f74dedc9ca3fe675