C2SoftAvcDec: Use a macro to control keep threads active flag

This change enables the keep threads active param
through a macro which makes it easier to change
when needed

Bug: 289097296
Test: atest android.mediav2.cts
      atest android.media.decoder.cts
      atest VtsHalMediaC2V1_0TargetVideoDecTest

Change-Id: I786716ce841812bd9d6d514a2ab61d00d5ad632e