webm: skip CSD for video

CodecPrivate would be handled as kMkvCodecPrivate by WebmWriter.
so, if there is CSD flag, it should be ignored.

Bug: 335614901
Bug: 342651775

Test: atest CtsMediaV2TestCases:MuxerTest
      atest CtsMediaV2TestCases:MuxerUnitTest
      atest CtsMediaV2TestCases:EncoderColorAspectsTest

Change-Id: I41f3ceafd943d21536b62b119caa4fcb66c219d2
Signed-off-by: Taehwan Kim <t_h.kim@samsung.com>