SyncSM8.1 always call transitionTo under processMessage

As state transitions always occur after a message has been processed,
calling transitionTo only under processMessage makes the flow easier
to understand and less error-prone.

The risk of this change is very minimal since it is only about
handling the error, but tethering will stop regardless.

Test: atest TetheringTests

Change-Id: I56c6cf6cc989464ee84a8333ac131afc808a3d95