[Cashtab] [extension address passing p2] Get wallet address in ecash: format from extension version of App.js

Summary:
T2550

Depends on D12314

In the browser extension version of Cashtab, get the active wallet's address in ecash: format.

Test Plan:

cd web/cashtab
npm run extension

Navigate to brave://extensions
Load unpacked (extension/dist folder) or click "Update" if you have already done this in local extension testing
Open the extension in its own tab (click the pop-out icon in top right corner of extension)
Open the dev console
Refresh the page and confirm the active wallet's address appears in the dev console, e.g. 'Address fetched from extension <activeWalletAddress>'
Change the active wallet
Note that the dev console msg now shows the new address

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12315