Update Addons url in fetch_allowed_addons.py
In android-components@267cb52a (https://github.com/mozilla-mobile/android-components/issues/9186) Mozilla moved from https://addons.mozilla.org to https://services.addons.mozilla.org
The stated reasoning of the move is caching on service.amo, so this isn't critical, but we should should follow Mozilla's lead.
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