docs: Adding file to support staging reference docs

We use the devsite tool to stage reference docs for review (to
go/dac-stage). That tool is currently broken, because it's expecting
to find a _reference-head-tags.html file which currently exists in
Piper but not in Gerrit. Copying that file into Gerrit lets us
build the Javadocs and stage them. (Files in p/f/b/docs don't
generally affect anything outside of the doc builds.)

Tested by building docs and staging Adapter.html to:
go/dac-stage/reference/android/widget/Adapter.html

Test: make ds-docs
Bug: 71717397
Change-Id: I05430f2d9f23e62b4423b2d6f304c4be1c43b880