Fix some issues during test

Fix some issues during test

- fix freezing error during seek
 : old packet is submitted
 : when overrun is signaled, underrun is not signaled because media is paused during seek
    so using g_cond_wait_until, continuously inserting packets
 : sometimes process is blocked in mediademuxer_read_sample() because of gst_app_sink_pull_sample()
    so split demuxer into audio and video demuxers
- fix crashing error during quit program(deinit)
 : when some stream is stopped, other stream is blocked in mediademuxer_read_sample during waiting for full
    so unselect track to stop appsink

Change-Id: I951bf0cae97735a1191adb4bd7eba2a267eb4707
Signed-off-by: Youngwoo Cho <young222.cho@samsung.com>