Test: Replace manual code with generated toString method

Binder already automatically implements toString() for parcelables.
The generated toString() returns the same string as what the below
hand-made methods return, except that "\n" is not replaced with "\\n".

  std::string toString(const std::vector<ResolverHostsParcel>& parms)
  std::string toString(const std::optional<ResolverOptionsParcel>& parms)
  std::string toString(const std::optional<DohParamsParcel>& params)
  std::string toString(const ResolverParamsParcel& parms)

This CL replace these hand-made toString() with binder-generated
toString().

Bug: 310112409
Test: resolv_integration_test passed
Change-Id: I47a95b50346021a998f755fdbc3fbe4b4b2b6116