Issue #2191273 by mfernea: Fix Feed typehints in aggregator module.

core/modules/aggregator/aggregator.module

core/modules/aggregator/aggregator.theme.inc

...regator/lib/Drupal/aggregator/Plugin/FetcherInterface.php

...gregator/lib/Drupal/aggregator/Plugin/ParserInterface.php

...gator/lib/Drupal/aggregator/Plugin/ProcessorInterface.php

...l/aggregator/Plugin/aggregator/fetcher/DefaultFetcher.php

...pal/aggregator/Plugin/aggregator/parser/DefaultParser.php

...gregator/Plugin/aggregator/processor/DefaultProcessor.php

...egator/lib/Drupal/aggregator/Tests/AggregatorTestBase.php

...aggregator_test/Plugin/aggregator/fetcher/TestFetcher.php

...l/aggregator_test/Plugin/aggregator/parser/TestParser.php

...egator_test/Plugin/aggregator/processor/TestProcessor.php