BUG-865: Move Immutables

Immutables is a utility class, so it should go into util. Its
capabilities can be expanded there to account for guava classes.

Change-Id: I6f93befd03249cb83a842a53600260b776822ba2
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-865: Move Immutables"