Make MotionClassifier 'final'

MotionClassifier's destructor invokes requestExit(), which then
invokes the virtual method reset().  If MotionClassifier had a
child class, this call to reset() from the destructor would
not use the dynamic binding, and thus wouldn't invoke any
child class's reset() override.

To avoid this potentially buggy and confusing situation, we
preemptively declare MotionClassifier 'final', which prevents
it from having children classes in the first place.

Test: TreeHugger
Change-Id: Id78cfe657b4be952b629bc43ce99b6927c833f05