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