Catch ArrayIndexOutOfBoundsException in StateMachine.toString()

StateMachine.toString() calls mSmHandler.getCurrentState(). In the
rare case when mStateStackTopIndex is -1 due to the StateMachine being
not yet initalized, this results in an ArrayIndexOutOfBoundsException.

Bug: 124858416
Test: none

Change-Id: Id558be95b6e12d8a923d325bca5f6f6d7f942b22