Fix #1028: Gson.getDelegateAdapter not working correctly for @JsonAdapter field

[Marcono1234]

Previously TypeAdapterFactory classes specified using the @JsonAdapter annotation for fields were only be able to get type adapters created by the enum and reflection-based factories when calling Gson.getDelegateAdapter(...).
This pull request changes it so they are able to get type adapters created by all factories registered on the Gson instance.

The behavior for @JsonAdapter on types (in contrast to fields) remains unchanged.

Note that this is an backwards incompatible change in case someone relies on Gson.getDelegateAdapter(...) returning a reflection-based type adapter. However, this might be a rare corner case and it appears keeping this behavior while also fixing #1028 is not possible.

Also note that this solution breaks if the created TypeAdapterFactory leaks a reference to itself, e.g. by storing a reference to itself in a static field or adding it to a static collection, and then later calling getDelegateAdapter. Or if TypeAdapterFactory.create(...) calls getDelegateAdapter from a separate thread. Though both cases are likely rare corner cases.

[Marcono1234]

This is necessary because jsonAdapterFactory.isFieldAdapterFactory would throw a NullPointerException otherwise.

This slightly changes the behavior in that previously null was treated as jsonAdapterFactory, which was however not documented and also likely not intended.

[Marcono1234]

Maybe instead of tracking adapters which have been created for fields annotated with @JsonAdapter; instead track adapters which have been created for classes annotated with @JsonAdapter and decide based on that which adapters to skip in getDelegateAdapter.

This would then allow custom reflection based adapters to support @JsonAdapter and proper usage of getDelegateAdapter as well, which would not be possible with the changes proposed here.

Though both that implementation and this implementation here would behave incorrectly when a custom InstanceCreator has been registered which returns a singleton factory instance, which is also registered as regular factory. (The current Gson implementation would not handle that correctly either.) But that is probably negligible because if @JsonAdapter on the class was called in the first place, then that factory singleton returned null before (since @JsonAdapter on class is called after custom factories), so it would not make sense that this singleton factory is specified for the @JsonAdapter (because it would return null again).
(That logic does not apply for @JsonAdapter on fields, but maybe there it is acceptable that getDelegateAdapter skips past the singleton factory, even though the call happened from a different context, that is from @JsonAdapter; and this is how Gson would behave currently anyways.)

[Marcono1234]

Has been superseded by #2435