Mark setter parameters for oneof fields with @NonNull

[Egorand]

Based on #1210.

// Java
public Builder foo(Integer foo) {
  this.foo = foo;
  this.bar = null;
  this.baz = null;
  return this;
}

Passing null as the argument will break the oneof contract, marking the parameter with @NonNull can at least hint at the potential problem.