Set threshold for silence detection defaults to 256

(closes issue #15685)
Reported by: david_s5
Patches:
      dsp-silence-threshold-init.diff uploaded by dant (license 670)
      issue15685.patch.v5 uploaded by pabelanger (license 224)
Tested by: danti

Review: https://reviewboard.asterisk.org/r/670/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@268690 65c4cc65-6c06-0410-ace0-fbb531ad65f3