common: Annotate Guava's ImmutableMultimap.of()

common: Annotate Guava's ImmutableMultimap.of()

All variants of the .of() method return a new multimap, so they
are necessarily @NonNull.

The parameters can also be marked @NonNull, because the map
doesn't support null keys or values, just like ImmutableMap.

Change-Id: I959449e609fe44b6571cca861ee40168583eb4f6
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/67182
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>