UKM client id generation code mixes Uint64 and Int64.

Relevant code:

    client_id = base::RandUint64();
  pref_service->SetInt64(prefs::kUkmClientId, client_id);

https://cs.chromium.org/chromium/src/components/ukm/ukm_service.cc?rcl=f3463c78b77bc4cefd8e748c4f47a2324515ad47&l=34

So we're generating and storing it in memory as a uint64_t but when we store it in prefs, we store it as Int64. This is confusing for no good reason and I guess compiler is silently ignoring the signed conversions.

Ideally, we should use SetUint64 on pref_service, but now that would require migrating prefs. We could instead make the rest of the code uint int64 at least.