Fix race condition in ProcessMonitor

If any new external process was enabled/disabled during
the process monitoring loop time, a RuntimeError: dictionary
changed size during iteration was thrown. This is fixed
by pre-building the service list from the dictionary
for the iteration.

Closes-Bug: #1401042

Change-Id: Id0a3af22fb85992bde35d8c691bee3ddf435639d
Merge "Fix race condition in ProcessMonitor"