tracing: Only compare timestamps from the same sequence

There are two different sequences in the test:
1. The sequence emitted by the tracing service.
2. The sequence emitted by the thread in the test.

The test tries to check that the clock snapshots are monotonic, but it
doesn't make sense to check two clock snapshots of two different
sequences, which will very likely be reordered (perfetto tries to read
a whole sequence before reading the next).

This fixes a failure in CI (msan, I could not reproduce locally) with
the error message:

```
[00:03:13] [ RUN      ] PerfettoApiTest/PerfettoApiTest.LegacyTraceEventsAndClockSnapshots/System
[00:03:13] [777.777] ng_service_impl.cc:1075 Configured tracing session 1, #sources:1, duration:500 ms, #buffers:1, total buffer size:1024 KB, total sessions:1, uid:1337 session name: ""
[00:03:13] ../../src/tracing/test/api_integrationtest.cc:4944: Failure
[00:03:13] Expected: (last_clock_ts[id].ts) <= (ts), actual: 4930745079518161 vs 4930745077415448
[00:03:13] This sequence:2 prev sequence:1 clock_id:6
[00:03:13] [  FAILED  ] PerfettoApiTest/PerfettoApiTest.LegacyTraceEventsAndClockSnapshots/System, where GetParam() = 2 (35 ms)
```

Change-Id: If493a2d45ff6258f23403bca072a1f58308f31ce