Make sure we've finished loading settings before processing any reloads

- Effectively throttles case where many reloads are sent in rapid
succession.

- Simplify Handler state machine by making the state explicit instead
of implicit in the sequence of messages.

- To guard against cascading timeouts due to RAM pressure or other
systemic issues, only start additional services if there is at most one
timed out setting at a time.

- Add some log messages

- Fix b/10487253

Change-Id: Ibe21533f7b644bbadf5b56b61ca0e28b49192470