C2SoftMpeg2Dec: Fix KEEP_THREADS_ACTIVE issue

Ensures proper handling of threads and mutex locks

Bug: 343793479
Test: atest MctsMediaV2TestCases
      atest MctsMediaDecoderTestCases
      atest MctsMediaCodecTestCases
      atest VtsHalMediaC2V1_0TargetVideoDecTest

Change-Id: I80a5d57b8942ac3a5d968de6df9e983191fa221d
1 file changed