Show explicit log message when ads are capped due to user activity

[btlechowski]

In brave/brave-core#6703 user activity frequency capping was implemented.
However a generic log message is shown when ad is capped. It is hard for QA to debug what happens.

Steps to Reproduce

1. Clean profile on staging
2. Enable rewards
3. Wait till ads are initialized
4. Trigger an ad

Actual result:

A generic ad message is shown in the logs
[ads_impl.cc(876)] Ad notification not shown: Not allowed based on history

Expected result:

An explicit message, for example:
Ad notification not shown due to user activity frequency capping

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Brave	1.16.46 Chromium: 86.0.4240.55 (Official Build) nightly (64-bit)
Revision	a6d625ef6f7fe8ea0675f1cf759155a05ee1be40-refs/branch-heads/4240@{#953}
OS	Ubuntu 18.04 LTS

cc @brave/legacy_qa @tmancey

[tmancey]

Closing issue as wont fix. Discussed in DM.