Add annotations for Map.keySet() and .values()

Add annotations for Map.keySet() and .values()

The returned Set and Collection always exist (would be empty
if there are no entries in the map), so can be marked @NonNull.

The generic types K or V keep the same modifiers as the map definition.

Change-Id: If826108fbbd9abb19c7f87f9ef6e27c047425461
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/59423
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>