combine wait for exec log with service exit log

- allows easier tracking of wait time from monitoring tools
- this change also reduces unnecessary log spam
- service exit log looks like this:
  init: Service 'exec 4 (/system/bin/otapreopt_slot)' (pid 611) exited with status 0 waiting took 0.060771 seconds

bug: 37752410
Test: reboot and check log
Change-Id: I122902538697f33939eede548e39f155ec419e03