Invoke register callback once during tts_prepare()

Invoke register callback once during tts_prepare()

Current code tries to create new notify callback handle every callback register. It is for
matching business logic for dbus. However, server can send message through expired callback handle
by this logic.

This patch makes code create and register callback handle once during tts_prepare().
By this change, server always sends message throught valid callback handle.
However, if registering callback is failed, this logic can not handle the situation.

If the situation occurs, handling code should be required.

Change-Id: I5e88be20ffc84c0ace99e4559440219c21d36089
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>