I am merely a user, not a contributor yet. But I would say a solid base, and not a full implementation of your wanted features list, is in OpenKeychain.

http://www.openkeychain.org/

Projects like this are why I continue to use Fdroid, Cyanogen AOSP, and believe the open source movement on mobile is worth the hassle. Oh yea, that and Password Store.

https://github.com/zeapo/Android-Password-Store/