Prefer InstanceIdentifier imports

InstanceIdentifier/YangInstanceIdentifier are now separate, so we can
import them at the same time. Do precisely that.

Change-Id: I54a5cd49ef7b8584a5b09a12ab97e132fb92cb57
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Prefer InstanceIdentifier imports"