Invokable no longer inherits from java.lang.reflect.AccessibleObject.

[copybara-service]

Invokable no longer inherits from java.lang.reflect.AccessibleObject.

There was no strong reason for this inheritance. Additionally, it is an accident that AccessibleObject is subclassable at all outside its class; its constructor is protected but should really have been package-private. That's an API mistake that was made in Java 1.2 and is hard to correct now, but the protected constructor is being deprecated. (My related rant from 15 years ago is here.)

Invokable also no longer implements GenericDeclaration.

Invokable does continue to provide instance methods with the same signatures as the ones inherited from AccessibleObject and GenericDeclaration, as they stood in Java 8.

This change is technically a breaking API change. In principle users could have assigned an instance of Invokable to a variable of type AccessibleObject. No code does that in Google's giant code base so it seems unlikely in practice. (In fact we only have a couple of dozen uses of Invokable overall.) Also, this API is @Beta so a hypothetical-but-very-unlikely breakage is acceptable.

RELNOTES=Invokable no longer inherits from AccessibleObject or GenericDeclaration, though it continues to define instance methods with the same signatures as the formerly-inherited ones. This is technically a breaking API change to this @Beta API. We think it very unlikely that anyone is affected in practice.