Add **/package.html to docs srcs

It is being used implicitly at the moment, make it explicit instead.

r.android.com/1305139 added this to the other droidstubs.

Bug: 153703940
Test: diff soong intermediates for framework-doc-stubs before and after
Change-Id: I708cfd02b9096b0245fddbbfb46893d912edd197

ApiDocs.bp

@@ -70,6 +70,7 @@ stubs_defaults {
         ":i18n.module.public.api{.public.stubs.source}",
         "test-mock/src/**/*.java",
         "test-runner/src/**/*.java",
+        "**/package.html",
     ],
     libs: framework_docs_only_libs,
     create_doc_stubs: true,