On-device feedback showed the equalizer's preset-cycling custom action
looked dead: many head units render custom actions icon-first, and a
monochrome icon cannot legibly encode "which of six presets" the way a
browsable list's text rows can.
This adds an "Ecualizador" folder to the car's browse tree, listing
"Desactivar" first, then the six factory presets by name, with the
currently-active one marked. Selecting a preset routes through the same
playFromMediaId seam every other browse-tree leaf already uses; picking
a preset while the equalizer is off turns it on and applies that preset.
Supersedes the earlier "no equalizer folder" rule (commit 2403da3),
which predated this feedback -- see decision auto/ecualizador-diseno.
The preset-cycling custom action still coexists with the folder in this
commit; it is removed in the next one.
