Add Log.wtf when transitionTo is used improperly

Using transitionTo in exit/enter (except in the terminal state) is
documented as undefined behavior and may cause unexpected results.
The current implementation appears to finish the current transition and
then transition to the new target state.

TEST=flash and play with the phone, no sign of immediate WTFs

Change-Id: I38a34b85c43d53c51514339587fc1269a069a454