[Linux] fix method response memory leaks (#159)

`fl_method_success_response_new()` does not take ownership of the value
but increments its reference count by one. Using `g_autoptr` is an
"easy" way to ensure that the newly allocated values get unreferenced
at the end of the scope and that way released as appropriate when the
success response is done with them.

Co-authored-by: LiJianying <[email protected]>

Author: jpnurmi