GDBusProxy: factor out async_init_data_set_name_owner

GDBusProxy: factor out async_init_data_set_name_owner

This removes the need for async_init_get_name_owner_cb to cope with being
called without a real GAsyncResult, and will simplify the addition of
correct thread-locking.

In async_init_data_set_name_owner, use the name_owner parameter instead
of the corresponding member of GDBusProxyPrivate, partly to reduce
pointer-chasing but mainly to avoid needing to hold the lock.

Signed-off-by: David Zeuthen <davidz@redhat.com>