Fix a class loading deadlock involving TypeAdapters.

[eamonnmcmanus]

Fixes #2739.

[Marcono1234]

Thanks a lot for noticing and troubleshooting this! It was also my fault for not paying attention to this during the refactoring.

In addition to your changes here, we could probably make it even safer by removing the JsonElementTypeAdapter.FACTORY field and instead using the newTypeHierarchyFactory call directly as initializer for TypeAdapters.JSON_ELEMENT_FACTORY, e.g.:

public static final TypeAdapterFactory JSON_ELEMENT_FACTORY =
    newTypeHierarchyFactory(JsonElement.class, JSON_ELEMENT);

That would then completely remove the direct cyclic dependency between JsonElementTypeAdapter and TypeAdapters.

(Also, at least in my opinion, we could also un-deprecate the fields in TypeAdapters again and remove the Javadoc. I think it was only useful in the situation where those fields were not used internally by Gson anymore.)

[eamonnmcmanus]

Thanks @Marcono1234, those changes seem reasonable.

[Marcono1234]

Really minor: You can omit the TypeAdapters. qualifier here (unless you added it intentionally) since this is already within TypeAdapters.

Suggested change

	TypeAdapters.newTypeHierarchyFactory(JsonElement.class, JSON_ELEMENT);
	newTypeHierarchyFactory(JsonElement.class, JSON_ELEMENT);

[cpovirk]

Sorry, this wasn't actually that complicated, but I needed to find a minute when I wasn't half-thinking about something else at the time.