Stop feedback thread only when stop is requested

Stop feedback thread only when stop is requested

- Issue:
Sometimes, the feedback thread plays the TTS feedback data for previous
request.

- Solution:
The issue is because feedback thread is exited before the engine
finishing to send TTS feedback data. Thus, some data can be remained in
feedback data queue even if thread is terminated.
This patch fixes termination logic for TTS feedback thread. Through this
patch, TTS feedback thread is terminated only when stop is requested,
and TTS feedback thread tries to consume all feedback data.

Change-Id: I8deaaab5f6b3e44e86b62d1ec3d8735e30a54780
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>