[cashtab] Add support for BIP21 URI as intent

Summary:
For now only limited to browsable intents, which is the one used by the system camera. This makes it possible to launch cashtab (Android app) from the system camera. If both cashtab and marlin are installed, android prompts the user to select which one to use.

This works both when the app is already launched or not. It jumps to the send screen with the proper content pre-filled.

Upon success or rejection, jumps to the home screen (on desktop or extension it closes the window).

Test Plan: Use the phone camera to scan an address qr code, or a paybutton qr code. Check it opens cashtab on the send screen (try both cold and warm start).

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D19603