apps/app_amd.c: Fixed total time and silence calculations

Between Asterisk 11 and Asterisk 13 there was a significant increase
in the number of AST_FRAME_NULL frames being processed by app_amd.c's
main loop. Each AST_FRAME_NULL frame was being counted as 100ms
towards the total time and silence. This may have been accurate
when app_amd.c was orginally added, but it is not in Asterisk 13.
As such the total analysis time and silence calculations were way
off effectively breaking app_amd.c

* Additional debug messages were added
* AST_FRAME_NULL are now ignored

ASTERISK-27610

Change-Id: I18aca01af98f87c1e168e6ae0d85c136d1df5ea9