Add reproducible builds verification notes for Android to our verifying signature page

On https://www.torproject.org/docs/verifying-signatures.html.en#BuildVerification we outline how to make a link between the bundles we actually ship (including update files) to the artifacts one gets by following our reproducible builds path.

So far, this contains instructions for Linux and Windows bundles. macOS is tricky and dealt with in legacy/trac#18925 (moved).

This ticket is to add respective instructions for our .apk file(s) we ship.