app_amd: Fixed timeout issue

app_amd: Fixed timeout issue

ASTERISK_28143 attempted to fix an issue where calls with no audio would never
timeout. It did so by adding AST_FRAME_NULL as a frame type to process in its
calculations. Unfortunately these frames seem to show up at irregular time
intervals. This resulted in app_amd returning prematurely most of the time.

* Removed AST_FRAME_NULL from the calculations
* Added a check to see how much time has actually passed since app_amd began

ASTERISK-28608

Change-Id: I642a21b02d389b17e40ccd5357754b034c3daa42