`@Nullable` doesn't work correctly with nested types

[eamonnmcmanus]

If the @AutoValue class has a property like this:

  abstract Map.@Nullable Entry<?,?> bar();

then auto-value-gson incorrectly generates a constructor like this:

  Foo(@Nullable Map.Entry<?,?> bar) {...}

It needs to use the same Map.@Nullable Entry syntax.

I am preparing a PR to fix this.