Modify the ProcessMonitor class to have one less config parameter

It's a follow up patch, as agreed on the ProcessMonitor review
patch to coalesce the check_child_processes parameter into
check_child_process_interval.

When this parameter is set to 0, the feature is disabled.

Change-Id: I2d4d8c6a6b7c17d42d8455c98968f75bcefbb689
Closes-Bug: 1371705
Merge "Modify the ProcessMonitor class to have one less config parameter"