[Cashtab] [extension address passing p7] Actually fetch the address from storage in the background.js function
[Cashtab] [extension address passing p7] Actually fetch the address from storage in the background.js function

Summary:
T2550

Depends on D12319

In background.js, get the requested address from the extension storage API

Test Plan:
```
cd web/cashtab
npm run extension
cd web/cashtab-components
npm run storybook
```
Navigate to `brave://extensions`. Click 'update' and click 'Inpsect views in background page'

Click 'Get Address' in your storybook tab. In the background.js dev console, confirm the active wallet address appears i.e. `background.js fetched address from extension storage, <yourAddress>`
Change the active extension wallet and try again. Confirm the new address appears in the background.js dev log

Note: if a user has not yet opened the latest version of the extension that saves this address, then it will be `undefined`. This is expected behavior.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12320

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