SoftVorbis: handle flush as well as flush with CSD

Previous fix added support for flush + CSD, but removed support for
flush without CSD.

Bug: 116510364
Change-Id: I85d8aa606b4bfde4511bafb0604d14b36b4d8827
Related-Bug: 113983576
Test: play issue_clip.webm
Test: atest com.google.android.exoplayer.gts.DashTest\#testVp9AdaptiveWithRendererDisabling