Destroy PDM signal before connected slots

:Release Notes:
Fix crashing of db8 when it stops

:Detailed Notes:
Explicitly destroy PDM signal

:Testing Performed:
Compiled and tested locally

:QA Notes:

:Issues Addressed:
[GF-46421] Crashing of db8 when it stops

Open-webOS-DCO-1.0-Signed-off-by: Maksym Sditanov <maxim.sditanov@lge.com>

Change-Id: I8b553a78e19950534f8ff0d4599d76e1ce32e231
Reviewed-on: https://g2g.palm.com/3913
Reviewed-by: Build Verification
Reviewed-by: Maksym Sditanov <maxim.sditanov@lge.com>
Tested-by: Maksym Sditanov <maxim.sditanov@lge.com>
Reviewed-by: Igor Kuksyuk <ikuksyuk@luxoft.com>