[Tts/Stt] Use thread lock only when add and remove event handler (#2782)

[Tts/Stt] Use thread lock only when add and remove event handler (#2782)

* [Tts] Use thread lock only when add and remove event handler

Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>
* [Stt] Use thread lock only when add and remove event handler

Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>
* [Tts/Stt] Remove useless thread lock statements

null conditional operator already guarantees the thread safety.
So it does not need additional thread lock.

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