GpuWork: fix gpu_work_period tracepoint

The tracepoint design was too fine-grained, too complex, and was missing
a gpu_id field.

- Fix the gpu_work_period tracepoint struct:

  - More coarse-grained.

  - Add gpu_id field.

  - Remove frequency and other fields.

- Update the documentation comment, adding more detail.

- Update the eBPF program and map structs to account for the tracepoint
  changes.

  - Aggregate the `total_active_duration_ns` and
    `total_inactive_duration_ns`.

  - Collect data for each (gpu_id, uid) pair.

- Update GpuWork class to account for the tracepoint changes.

  - Remove references to "frequency"; we now collect "gpu work".

  - Rework dumping code; dump active and inactive time.

  - Rework stats upload:

    - Log active and inactive time.

    - Log per (gpu_id, uid) pair.

    - Reduce the amount logged if we observe a large number of GPUs.

    - Remove UIDs with a very low GPU time.

Bug: b/221096545
Change-Id: I33d1c55f886913dad7b563e7b9a25cc1af0308f2