System/OS ad notifications are missing icons

Description

System/OS ad notifications are missing icons

Steps to Reproduce

1. new profile
2. launch Brave using --enable-logging=stderr --vmodule="*/variations/*"=6,"*/bat-native-ledger/*"=6,"*/brave_rewards/*"=6,"*/bat-native-ads/*"=6,"*/bat-native-confirmations/*"=6,"*/brave_ads/*"=9,"*/brave_user_model/*"=6 --brave-ads-staging --brave-ads-debug --rewards=staging=true,reconcile-interval=3
3. open a new-tab page
4. click on the Start using Rewards button
5. open 3 or so more tabs
6. wait a bit
7. move the mouse/keyboard
8. look at the system-level Brave Ads ad notification

Actual Result: It’s missing its icon/favicon

|

Expected Result:

Should have its respective icon/favicon

BTW: custom-ad notifications are fine 👍

Reproduces how often:

100%

Brave version (brave://version info)

Brave	1.31.46 Chromium: 94.0.4606.41 (Official Build) nightly (x86_64)
Revision	333e85df3c9b656b518b5f1add5ff246365b6c24-refs/branch-heads/4606@{#845}
OS	macOS Version 12.0 (Build 21A5506j)

cc @kjozwiak @tmancey @btlechowski @GeetaSarvadnya

[LaurenWags]

labelling as QA/Blocked until uplift is completed and an RC containing this fix is created so it can be tested.

[bsclifton]

cc: @simonhong