[Cashtab] Change the app id to com.cashtab.app

Summary:
The current app id is "org.bitcoinabc.cashtab"

The standard is for the app id to be reverse URL. Since cashtab lives at www.cashtab.com, a normal ID here could be com.cashtab.www

However looking at other crypto apps, it seems standard to use app instead of wwww (e.g. phantom does this)

I think tying to the cashtab domain for the cashtab app is appropriate, as the app will always have some kind of presence at cashtab.com

Test Plan:
npm test

i built a test release and the play store recognizes the new app id, tho now there is some rigamarole in updating it there

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18936