Stations and tracks with no artwork showed empty tiles in the car. The
browse tree's itemEmisora/_itemLocal already fell back to the rotating
station_art_* drawable via artUriPara/artUriLocal, but the "now playing"
MediaItem built when actually playing something (car tap, phone-initiated
play, folder-queue advance, direct local-track tap) did not, so the car's
now-playing screen still went blank.
Reuse the SAME artUriPara/artUriLocal fallback (already the project's one
selection scheme, mirroring PluriStationArtFallback) at every "now playing"
construction site: reproducirPorMediaId, ServicioAudio.reproducir (now via
the extracted, unit-tested mediaItemParaEmisora), construirMediaItemColaLocal
and reproducirPistaLocal.
Guard the reverse direction too: emisoraDesdeMediaItem (extracted from the
handler's private method, now unit-tested) only reflects artUri back into
Emisora.favicon when it passes faviconUsable, so the phone UI's
CachedNetworkImage widgets never attempt a doomed fetch of the car's
android.resource:// fallback URI -- they keep falling back to
PluriStationArtFallback exactly as before.
