Declare Visualizer::setEnabled() 'final'

We call the virtual method setEnabled() from Visualizer's
destructor.  In the destructor, the virtual binding is not used.
This isn't a problem for any current code, but to future-proof
against a possible future subclass of Visualizer, we declare
this method 'final'.

Test: TreeHugger
Change-Id: I703580b37cc1bf7f411caa723f95a691dff840a5