[Cashtab] Stop enumerating cameras so we open scanner faster

Summary:
We do not get any benefit from checking all the cameras, since we know the one we want anyway.

figured this out when working on the capacitor diff, it's working there

Test Plan: npm test, scan with cashtab.io

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18497