C2SoftMpeg2Dec: keep threads active for entire sequence

Bug: 289097296
Test: atest android.mediav2.cts
      atest android.mediapc.cts
      atest android.media.decoder.cts
      atest android.media.encoder.cts
      atest android.media.codec.cts
      atest android.video.cts
      atest android.videocodec.cts
      atest VtsHalMediaC2V1_0TargetAudioDecTest
      atest VtsHalMediaC2V1_0TargetAudioEncTest
      atest VtsHalMediaC2V1_0TargetVideoDecTest
      atest VtsHalMediaC2V1_0TargetVideoEncTest
      atest VtsHalMediaC2V1_0TargetComponentTest
      atest VtsHalMediaC2V1_0TargetMasterTest

Change-Id: I786716ce841812bd9d6d514a2ab61d00d4ad522e