Annotate java.util.Set.

You can see the source files with the annotations in
https://android-review.googlesource.com/c/platform/libcore/+/699804
(which obviously is not intended for submission). The .jaif changes
here were copy-and-pasted from the results of running the
extract-annotations tool on those sources.

Test: make docs
Bug: 64930165
Change-Id: Icdc220fa48285ddf8319e3a901eeea1c71e60709