Update/optimize calls to advertisers countries

[budzanowski]

Changes proposed in this Pull Request:

Closes #800 .

We have been notified that there are too many calls to the advertisers/countries endpoint. This PR helps to reduce the number of calls via two things:

1. Increase the time for which the response is cached from 1 to 2 days. This endpoint is basically static so we could increase the number even higher if we would be forced to.
2. Detect if we are connected and if not abort the API call - go straight to the fallback list.

Detailed test instructions:

The plugin should not be connected.

1. Set a breakpoint at Pinterest_For_Woocommerce_Ads_Supported_Countries::get_countries
2. Open the Marketing -> Pinterest page.
3. Call to the get_countries should not trigger the API call but the fallback should be used.
4. Connect the plugin
5. After connection the call to the get_countries should trigger an API call ( first time )
6. Refresh the page - the next call should use the cached value.

Changelog entry

Tweak - Don't fetch the allowed countries list when not connected.
Tweak - Increase allowed countries cache lifetime to two days.

[JPry]

Works as described, just needs the PHPCS fix and the one suggestion I made before merging.