ui: Make GPU plugin track group names consistent (#5369)

Drop the "GPU" prefix from sub-group names under the top-level "GPU"
group. "GPU Frequency" becomes "Frequency" and gpu_memory uses its
gpuTrackName ("Memory") for the fallback group instead of the DB track
name ("GPU Memory").

Memory and Frequency tracks follow the same hierarchy: single GPU puts
the track directly under the GPU group; multiple GPUs creates a group
(e.g. "Frequency") with per-GPU tracks ("GPU 0 Frequency", "GPU 1
Frequency") directly under it. "Counters" and "Hardware Queues" keep
per-GPU sub-groups since they have multiple tracks per GPU.