Follow up to #18523: Improve ads notification serving logging

[btlechowski]

Follow up to #18523: High CPU usage while Brave ads is enabled
The fix for above issue introduces a boolean flag which tracks if we are already serving an ad.
It could happen that the flag is not cleared therefor preventing users from getting ads.
The log will help us debug those scenarios.

Steps to Reproduce

1. Add logging:
   https://github.com/brave/brave-core/blob/100ab8e90be161e96333ca493e7d5f0342799cfa/vendor/bat-native-ads/src/bat/ads/internal/ad_serving/ad_notifications/ad_notification_serving.cc#L101

Brave version (brave://version info)

Brave	1.32.62 Chromium: 95.0.4638.40 (Official Build) nightly (64-bit)
Revision	e3e7c76ba0284b16087cf4cf3153abfaef6470c7-refs/branch-heads/4638@{#624}
OS	Ubuntu 18.04 LTS

cc @brave/legacy_qa @rebron @tmancey @jsecretan

[tmancey]

Closing issue as will be resolved by #17542

[kjozwiak]

As per brave/brave-core#10613 (comment), the uplift PR was scaled back and just includes the logging component rather than all the code from brave/brave-core#10459 which is going to ride the trains via 1.33.x. as per the discussion on Slack.