Turn droiddoc's metalava_merge_annotations_dir into a list.

This change replaces droiddoc's metalava_merge_annotations_dir
parameter (which takes a single value) with
metalava_merge_annotations_dirs (which takes a list). This will makes
it possible to merge libcore annotations in from a separate directory
under ojluni, instead of adding them in a preprocessing step.

Test: `make metalava-api-stubs-docs` with a local change to add a second directory
Bug: 111639530
Change-Id: I8eb778038c1cb52ddb6fc86d972f555f49e836b2