config: Update keep_alive to support silence playback on non-hdmi devices

Keep_alive can now handle silence playback on multiple concurrent
devices such as primary output devices as well as HDMI

Change-Id: Icdf22c9d8d1264d463b23c8f54020e792840b73c
Signed-off-by: Md Mansoor Ahmed <mansoor@codeaurora.org>