stdlib: Wattson: Handle multiple forms of perf events (#5480)

Depending on whether the perf event comes from the Perfetto config or
from running simpleperf concurrently as the Perfetto trace, the data
format saved is different. Handle this.

If the perf event is from Perfetto config polling, then the counters
will be monotonically increasing. Example config below:

data_sources {
  config {
    name: "linux.perf"
    perf_event_config {
      target_cpu: 0
      timebase {
        poll_period_ms: 10
        raw_event {
          type: 13
          config: 43
        }
        timestamp_clock: PERF_CLOCK_MONOTONIC
        name: "arm_dsu_0-l3d_cache"
      }
    }
  }
}

If the counters come from concurrently running simpleperf, then the
table has counts per period. Example command below:

simpleperf stat -a -e arm_dsu_0/l3d_cache/ --interval 10

Bug: 503368151

Signed-off-by: Samuel Wu <wusamuel@google.com>