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.

It is implemented by passing the --merge-annotations argument to
metalava multiple times, which is already supported by metalava's
command line parsing.

Test: `make metalava-api-stubs-docs` with a local change to add a second directory in //frameworks/base/Android.bp
Bug: 111639530
Change-Id: I53d31f1dd45c13405b4511c2b44cbeb7f0e439d1