common: Modify glib threads to be unreferenced

common: Modify glib threads to be unreferenced

To trigger update, GThread is used and it is created by g_thread_try_new
function. This function returns GThread object(pointer), which is a
newly allocated resource. Since update-manager does not manage these
threads with join, this resource should be unreferenced.

To unreference GThread, g_thread_unref is called. Since the thread holds
its GThread reference, it is safe to unreference the GThread.

Change-Id: Id656b9404b21ce5d859356beea0717c2481bc724
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>