Exceptions from the default adblock engine should be applied to the additional engine

[antonok-edm]

Our way of protecting 1st-party content in default blocking has been to not check the default engine for first-party requests. But, it was discovered that if the default engine contains an exception, it isn't respected (because it's not checked at all). So we still need to check the default engine, but only to look for exceptions.

@ryanbr has been working around this for now by manually importing exceptions to Brave's first-party list. However, the browser should be able to handle this automatically.