Follow up to #17723: Notification ads are not shown on Linux

[btlechowski]

Follow up to #17723

System notifications are not shown

Steps to Reproduce

1. Clean profile
2. Run Brave with

--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

1. Enable rewards and ads
2. Open 3 new tabs
3. Trigger a notification ad

Actual result:

Ad is not shown
Logs show:

[3640:3640:0827/144154.543715:VERBOSE1:browser_manager.cc(86)] Browser did become active
[3640:3640:0827/144154.547834:VERBOSE6:user_activity.cc(41)] Triggered event: 01 (5.5:2:3600 s)
[3640:3640:0827/144212.001348:VERBOSE1:ads_impl.cc(204)] Browser state changed to idle
[3640:3640:0827/144221.013777:VERBOSE1:ads_impl.cc(221)] Browser state changed to unidle after 23 s
[3640:3640:0827/144221.014764:VERBOSE2:permission_rule_util.cc(23)] Notifications not allowed
[3640:3640:0827/144221.014930:VERBOSE1:ad_notification_serving.cc(104)] Ad notification not served: Not allowed due to permission rules

Expected result:

ad is shown

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Brave	1.31.12 Chromium: 93.0.4577.58 (Official Build) nightly (64-bit)
Revision	c4410ece044414ea42fa4ba328d08195e818a99c-refs/branch-heads/4577@{#1076}
OS	Linux

cc @tmancey @jsecretan @rebron @brave/legacy_qa

[btlechowski]

Verification passed on

Brave	1.30.68 Chromium: 93.0.4577.63 (Official Build) beta (64-bit)
Revision	ff5c0da2ec0adeaed5550e6c7e98417dac77d98a-refs/branch-heads/4577@{#1135}
OS	Ubuntu 18.04 LTS

Verified test plan from the description

Verified system notifications are shown by default

Verified custom ads notifications are shown when overridden through variations.