omx_comp: add namespace YamiMediaCodec

Signed-off-by: Tang Xin <xin.t.tang@intel.com>
Please do not use YamiMediaCodec:: everywhere.It's boring and not handy.  Just use "namespace YamiMeiaCodec {  }" to enclose all codes.
Like "namespace meida {}" in https://chromium.googlesource.com/chromium/src/media/+/37c295df72eeaca45fe8edcbba67153f95b0b357/filters/gpu_video_decoder.cc