Transcoder: Add HDR -> SDR unit test using sample plugin.

This commits adds a unit test that force enables loading
sample HDR plugin loading and tries to transcode and HDR10
video. If the codec supports the conversion the test checks
that the transcoded video is SDR. If the codec does not
support the conversion the test checks that transcoding
returns unsupported.

Bug: 180011717
Test: atest HdrTranscodeTests
Change-Id: I9cb26589d320744670b619faa4b35ce5c0541e91