After this change, all synchronous state machine functions are
implemented.
There are some intentional limiations:
1. Allow setup states before machine thread started or inside machine
thread but states can only be setup once before calling #start.
2. The behavior of transitionTo is defined so that the destination
state can never change during state transitions. Once a destination
has been specified, it cannot be changed until the previous state
transition is complete. In other words, SyncStateMachine does not
support calling transitionTo in State enter or exit.
3. Support sendSelfMessage in State#processMessage(), #enter() and
#exit(). This allow automaton to decide the follow up action by
itself. For example: if something wrong happens during state
transition, the implementation can enqueue an error message then
process the error message after finishing the current state
transition.
Test: atest SynStateMachineTest
Change-Id: I0790df4eeab2dccdb9f407d9131a62c3c12d123b
21cdb40e59