StateMachine: make toString() terser

The current implementation of the toString()
method calls dump(). This causes two problems:

1. toString() may return a large string. This
   is at odds with the advice in the documentation
   for Object.toString(), which says that the
   returned String should be concise, and easy
   to read.
2. The dump() method is overriden by many of the
   StateMachine subclasses. Some of those subclasses
   have dump() implementations that are expensive,
   and/or have dependencies on other objects.

To resolve these problems, we simpify
StateMachine.toString().

Along the way: remove a stale comment about
implementing dump() using toString().

Note: only ran the StateMachine tests, since some
other tests are already failing.

Bug: 36661851
Test: tests/utiltests/runtests.sh \
      -e class com.android.internal.util.StateMachineTest
Change-Id: I5c16c650f01178c4d018b6a65e4aa95fb905aff6