[merge][java] use merge sort
[merge][java] use merge sort

Summary: Since field/method/annot lists are sorted/unique, a version of merge sort could be much faster and allocate less than `List.dedup_sort (l1 @ l2)`

Reviewed By: skcho

Differential Revision: D33635969

fbshipit-source-id: 07ab64e1ce

facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer