Destroy tts handle always on the main thread

Destroy tts handle always on the main thread

- Issue:
In C# application, the garbage colletor can destroy tts handle on a sub
thread, and this behavior can make thread safety issues.

- Solution:
The C# garbage collector can release the instance on a sub thread, so
this can make race condition issues. To prevent this kind of issues,
this patch adds a new logic to tts_destroy() function. Through this
change, the handle destruction logic of tts_destroy() will always be
run on a main thread. This change will prevent race condition issues
because the logic insures that every handle destructions are run on a
main thread and they are not destroyed at the same time.

Change-Id: I90c248a4975410d4211dea93f24ea43e7c00ecb1
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>