video editor: refine frame skip disabling code

BZ: 65223

Patch 69193 doesn't work well sometimes. So use another
libmix interface to disable frame skip in low bitrate encoding.
Also fix some unintialized varaible in libmix encoder.
Signed-off-by: Elaine Wang <elaine.wang@intel.com>

Change-Id: Ied588d0613d94ab6106309c6d2975fbf4cf71978
Reviewed-on: http://android.intel.com:8080/72104
Reviewed-by: Wang, Elaine <elaine.wang@intel.com>
Reviewed-by: Gu, Wangyi <wangyi.gu@intel.com>
Reviewed-by: Shi, PingX <pingx.shi@intel.com>
Tested-by: Shi, PingX <pingx.shi@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>
2 files changed