Refactor Service::Start method.

This CL extracts code from Service::Start into four helper functions,
bringing Service::Start down to 134 lines vs 212 lines originally. This
makes the method a lot easier to follow. There is no change in behaviour.

Also, make error messages consistent (start with lowercase) and
format Service::Start to fit in 100 cols.

Bug: 30035168

Change-Id: If979976fba4d339a336d030f802ca9f169fd012c