Extend Android app to support receiving a read-cap

The deliverable is as above with the following additions:

1. Integrate with the Haskell magic-wormhole client
2. Use a user-typed code to access a shared magic-wormhole mailbox
3. Receive a folder-invite, including a Collective read-only capability
4. (optional) allow user to say Y/N to the invite (replying via mailbox as appropriate)
5. Store the Collective read-only capability as a folder locally

Note: Code and issue tracker is in https://github.com/LeastAuthority/haskell-magic-wormhole