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

docs/html/_shared/_reference-head-tags.html

+  <!-- Added to Gerrit so we can stage reference docs directly from
+       Android builds; this file should *not* be migrated to Piper -->
+
+  <meta name="top_category" value="develop" />
+  <meta name="subcategory" value="reference" />
+  <meta name="book_path" value="{% if book_path %}{{ book_path }}{% else %}/reference/_book.yaml{% endif %}" />
+  <meta name="project_path" value="/reference/_project.yaml" />
+  <meta name="page_type" value="reference" />