Extend Android app to support QR Codes

The deliverable is as above with the following additions. The magic-folder pairing protocol is extended to allow reading the wormhole code via QR code

1. Incorporate a QR Code decoding library
2. Set the application up to accept ps-pair: URLs
3. Parse the code out of such an URL (instead of user typing it in)
4. Hard-code all other parameters (AppID, wormhole mailbox address, version, ..)

Some initial pointers from Obsidian available in https://3.basecamp.com/4757487/buckets/31395255/card_tables/cards/5918123449#__recording_5918296121