Introduce the RunService() method

The Service::Start() method is so long that its length negatively
affects readability of the code. Hence this patch that splits
Service::Start().

Test: Booted Android in Cuttlefish.
Change-Id: I5a6f587ecc5e6470137de6cceda7e685bce28ced
Signed-off-by: Bart Van Assche <bvanassche@google.com>