common: Annotate TreeMultimap.create()

common: Annotate TreeMultimap.create()

All variants of this method return a new map, so it can be
annotated @NonNull.

Remove now-unnecessary checkNotNull() checks where this was called.

While at it, rewrote some comparator definitions in more succinct
lambda expressions.

Change-Id: Ic7a78eb38b9cf3fd2b8e79ab385d73c5d4cc2961
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/68106
Reviewed-by: Hudson CI