Add Soong target for convert libcore droiddocs.

And make annotated_ojluni_files generation as genrule

The droiddoc gen for frameworks depends on:
LOCAL_GENERATED_SOURCES:=$(framework_docs_LOCAL_GENERATED_SOURCES)
framework_docs_LOCAL_GENERATED_SOURCES come from ojluni_annotate_output.
In order to use ojluni_annotate_output in Soong properly, make the
annotated ojluni srcs generation as genrule in Soong.

Test: m -j checkbuild and check out srcjar content.
Bug: b/70351683
Change-Id: Ie2682625288bc86c9845177e94c765d591495db3