ui: Add multi-machine support to GPU track plugins (#5272)

Change the GPU track dimension from raw gpu number to resolved ugpu
(GPU table row ID), matching the upid/utid pattern. All GPU event
parsers now call GpuTracker::GetOrCreateGpu() to resolve gpu_id to
ugpu before creating tracks.

- GetOrCreateGpu returns tables::GpuTable::Id instead of void
- Dimension key renamed from "gpu" to "ugpu" in tracks_common.h
- Removed "gpu" dimension handler from track_tracker.cc since callers
  now resolve gpu_id to ugpu themselves
- Updated ftrace_parser, gpu_event_parser, gpu_work_period_tracker,
  and system_probes_parser to resolve gpu_id before track creation
- SQL views (gpu_counter_track, android_gpu_work_period_track) updated
  to use 'ugpu' key and join with gpu table for raw gpu_id

Also adds Gpu helper class (mirroring Cpu) and updates the GPU plugin
to use ugpu-based URIs, machine labels, and machine-aware grouping
for multi-machine trace support.

```
Single machine (unchanged):
  GPU
    Gpu 0 Frequency       [_____----____--]
    Counters
      CounterX            [____------__--_]
    Hardware Queues
      Queue 0             [event1] [event2]
    GPU Memory            [____-----------]

2 machines, each with 1 GPU:
  GPU
    GPU Frequency
      Gpu 0 Frequency (machine 1)   [___----_-------]
    Counters
      GPU 0 Counters (machine 1)
        CounterX                    [__--------__--_]
    Hardware Queues
      GPU 0 Hardware Queues (machine 1)
        Queue 0                     [event7] [event8]
    GPU Memory                      [____-----------]
```