Allow models to override @Nullable on third-party functional interfaces

[lazaroclapp]

This PR does two things, one uncontroversial and the other more opinionated

1. [Uncontroversial] It acknowledges LibraryModels.nonNullParameters() when deciding the nullability of the arguments in a lambda with a functional interface which otherwise marks the parameters as @Nullable. Before we did acknowledge this when calling the FI methods directly, but not for lambda expression based overriding.

2. [Opinionated] We override the nullability of Guava's Function and Predicate functional interfaces. This means that lambdas implementing these in our code will need to return @NonNull but need not check the incoming arguments.

This has implications for checking Guava code with NullAway, but not for treating it as annotated vs unannotated with restrictive annotations enabled.

cc: @kurtisnelson

[lazaroclapp]

This is actually a WIP. ToDo:

• Test cases
• Check here too: https://github.com/uber/NullAway/blob/master/nullaway/src/main/java/com/uber/nullaway/NullAway.java#L550

[shubhamugare]

• Check here too: https://github.com/uber/NullAway/blob/master/nullaway/src/main/java/com/uber/nullaway/NullAway.java#L550

What is to be done about this?

[lazaroclapp]

• Check here too: https://github.com/uber/NullAway/blob/master/nullaway/src/main/java/com/uber/nullaway/NullAway.java#L550

What is to be done about this?

See latest commit. The idea being that to compute if an unbound instance method ref can be passed to a method that expects a particular functional interface, one thing must be true: the first argument (the receiver for an instance method) must be @NonNull. We were checking this already, but what defined whether this parameter was nullable or not was 100% the bytecode annotations, without acknowledging the distinction between annotated and unannotated code, or any overrides due to handlers (including library models).

[msridhar]

Wish we had generics support to properly handle Function and Predicate but this looks fine for now

[navahgar]

LGTM