Reactivate sound ducking when background volume ratio is changed

Reactivate sound ducking when background volume ratio is changed

To avoid some side effect or unintended behavior, current code does not apply the change of
background volume ratio immediately. To apply it immediately, tts service has to deactivate the
sound ducking and activate it again. This may cause tick noise or failure to recover background
volume. However, the requirement wants to apply the background volume ratio immediately.

Thus, this patch deactivate and activate the sound ducking right after changing background volume
ratio. To do this safely, the new code handles the behavior for background volume on main loop.
And this new code only works when the sound ducking is activated.

Change-Id: I11ea051aaa69b5a0bd959853d72943569c678a57
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>