Refactor policy class lookup

What does this MR do?

Currently the lookup may check whether a class respond to declarative_policy_class multiple times, which can be avoided.

Since declarative_policy_class is a very specific method, I think it is safe to assume whoever take use of this will define the correct class name, therefore we don't have to rescue error when calling constantize.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.