P3A crash fix

[DJAndries]

Resolves brave/brave-browser#46549

1. UpdateMetricValue gets called by the HandleHistogramChange loop in P3AService::Init, which creates the map elements:

   brave-core/components/p3a/message_manager.cc

   Line 159 in c3e0933

   constellation_prep_log_stores_[*log_type]->UpdateValue(

2. Start gets called, which calls RemoveObsoleteLogs. All elements in json_log_stores_ will be null if JSON reporting is deprecated (which is the case, via Griffin). It tries to call MetricLogStore::RemoveObsoleteLogs on null pointers

[SergeyZhukovsky]

I DMed in slack, but the crash still happens with that change.

[iefremov]

if there is no obvious fix, we should revert the whole thing while investigating

[bsclifton]

PR we can revert if needed #28362

++ from me on reverting until there's a solid solution

[SergeyZhukovsky]

I verified the change in the current(updated) PR on Android and made sure there is no crash anymore.

[SergeyZhukovsky]

++ as I mentioned above, the changes are verified on Android and no issue there anymore.

[brave-builds]

Released in v1.81.50