Make sure PWMs are running in sync.

The 3 (red, green, blue) PWMs were sometimes running out of sync,
depending on the notification LED color flow and timing.

Make sure they are running in sync by
- adjusting the PWM atomically (that is, not adjust each one within its
  own worker task, where other tasks could get run inbetween)
- restarting the PWM if it was running before

Change-Id: Iebddc898c467c11dd412f239a8e26dd22f42c28b