Fixes #5866 - Reduced Product json serialization footprint

Activation Keys index call was loading all attributes of a Product using Product model's as_json call.
This lead to some serialization issues causing activation key list call to fail.
This commit intends to expose only the necessary attributes of products
related to the given activation key.
Merge pull request #4136 from parthaa/ak-fix

Fixes #5866 - Reduced Product json serialization footprint