Add native_handle_init

HIDL requires file descriptors to be wrapped in native_handle_t.  We want
a low overhead way to do that when the number of file descriptors is known
at compile time.  Instead of

  // wrap an fd in native_handle_t
  native_handle_t* fd_handle = native_handle_create(1, 0);
  if (!fd_handle) {
    // clean up and return error
  }
  fd_handle->data[0] = fd;

  hidl_cb(..., fd_handle);

  native_handle_delete(fd_handle);

this change adds native_handle_init to allow for

  // wrap an fd in native_handle_t
  NATIVE_HANDLE_DECLARE_STORAGE(fd_storage, 1, 0);
  native_handle_t* fd_handle = native_handle_init(fd_storage, 1, 0);
  fd_handle->data[0] = fd;

  hidl_cb(..., fd_handle);

Test: make libcutils
Bug: 32021609
Change-Id: If1fd07482243d37492fdea57c602a1b13c8953cc