Fix FPs with multiple initialization of Singletons

[JuditKnoll]

This PR fixes #2934.
The MultipleInstantiationsOfSingletons detector is a bit eagerly considers classes as singletons. With this change, the detector

• accepts enums with 1 elements as well, not only those with 0 elements,
• recognizes if instance field is assigned a specific inner child instance, not the containing class itself, and does not consider these classes as singletons,
• does not consider singletons abstract classes or interfaces,
• can find the instance getter method more accurately.