Add "Open-source licenses" to menu

As discussed on dev@taverna:

Add a menu item, something like

> Settings -> Help & feedback -> Open-source licenses

to show the LICENSE.txt and NOTICE.txt

including those we are required to propagate from third-party libraries bundled inside the .apk.

To check what we are required to propagate it should be sufficient to check the licenses of our dependencies and look if the inside of the JARs already have a NOTICE or LICENSE file or similar.

(E.g. an Apache-licensed JAR with no NOTICE file? Nothing more to do. A JAR with a BSD-3 LICENSE file? We need to show that)

Sadly tooling for this is still a bit abysmal, so we probably have to create that text file manually.