Follow up to #18846: Allow users to disable Custom Brave notification ad fallback from brave://flags

[btlechowski]

Follow up to #18846

It seems that disabling Allow Brave Ads to fallback from native to custom push notifications has no effect. Tried with AdNotifications->can_fallback_to_custom_notifications set to true and false

Steps to Reproduce

1. Run Brave with flag AdNotifications->can_fallback_to_custom_notifications set to true
2. Set AllowedToFallbackToCustomAdNotifications to disabled in brave://flags
3. Trigger an notification ad

Actual result:

Custom notification is shown

Expected result:

Native (System) notification is shown

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Brave	1.34.63 Chromium: 96.0.4664.110 (Official Build) beta (64-bit)
Revision	d5ef0e8214bc14c9b5bbf69a1515e431394c62a6-refs/branch-heads/4664@{#1283}
OS	Ubuntu 18.04 LTS

cc @tmancey @jsecretan

[btlechowski]

Verification passed on

Brave	1.57.27 Chromium: 115.0.5790.114 (Official Build) beta (64-bit)
Revision	18977fb43f693d5a8deeb48bd1cfa52651d3f73e
OS	Windows 10 Version 22H2 (Build 19045.3208)

Verified test plan from brave/brave-core#18581

Verified custom notification ad is shown when system notifications are disabled and --enable-features=NotificationAds:can_fallback_to_custom_notifications/true

Verified custom notification ad is shown when system notifications are enabled and --enable-features=NotificationAds:can_fallback_to_custom_notifications/true

Verified custom notification ad is shown when system notifications are enabled and AllowedToFallbackToCustomAdNotifications is disabled

---

Verification passed on

Brave	1.57.27 Chromium: 115.0.5790.114 (Official Build) beta (64-bit)
Revision	18977fb43f693d5a8deeb48bd1cfa52651d3f73e
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#18581

The custom notification ad was not shown at all. The unblinded token for view was redeemed
Probably due to #18799

[5895:5895:0727/195458.813271:VERBOSE1:eligible_notification_ads_v2.cc(38)] Get eligible notification ads
[5895:5895:0727/195458.825809:VERBOSE1:eligible_notification_ads_v2.cc(124)] 20 eligible ads out of 20 ads
[5895:5895:0727/195458.826255:VERBOSE1:notification_ad_handler.cc(168)] Opportunity arose to serve a notification ad
[5895:5895:0727/195458.826534:VERBOSE1:notification_ad_serving.cc(125)] Found 1 eligible ads
[5895:5895:0727/195458.826648:VERBOSE1:notification_ad_serving.cc(188)] Served notification ad
[5895:5895:0727/195458.826718:VERBOSE1:notification_ad_handler.cc(185)] Served notification ad:
  placementId: 1327cecb-071a-4e46-8436-60a8c4db5fab
  creativeInstanceId: 7f1a7980-cca5-4544-ac61-1a18737dc241
  creativeSetId: 47294a09-8053-4fb5-b171-531bec199a1e
  campaignId: 12e88c0d-fb1b-4316-ade0-cc06eb57227e
  advertiserId: 93130af3-2def-4ecb-b836-b3772e73b3c9
  segment: untargeted
  title: Fung - Wallet
  body: Click here for the NEW Brave Wallet
  targetUrl: brave://wallet/
[5895:5895:0727/195458.827281:VERBOSE6:ads_service_impl.cc(808)] Timeout notification ad with placement id 1327cecb-071a-4e46-8436-60a8c4db5fab in 120 s
[5895:5895:0727/195458.866788:VERBOSE3:notification_ad_handler.cc(196)] Served notification ad with placement id 1327cecb-071a-4e46-8436-60a8c4db5fab and creative instance id 7f1a7980-cca5-4544-ac61-1a18737dc241
[5895:5895:0727/195458.867224:VERBOSE9:client_state_manager.cc(476)] Saving client state
[5895:5895:0727/195458.873407:VERBOSE9:client_state_manager.cc(485)] Successfully saved client state
[5895:5895:0727/195458.884621:VERBOSE3:notification_ad_handler.cc(205)] Viewed notification ad with placement id 1327cecb-071a-4e46-8436-60a8c4db5fab and creative instance id 7f1a7980-cca5-4544-ac61-1a18737dc241

[5895:5895:0727/195505.865394:VERBOSE1:redeem_opted_in_confirmation.cc(361)] Successfully redeemed opted-in view confirmation for ad_notification with transaction id 04e24cf8-f27b-4a9b-97a4-bf51e57a9f17 and creative instance id 7f1a7980-cca5-4544-ac61-1a18737dc241

---

Verification passed on

Brave	1.57.27 Chromium: 115.0.5790.114 (Official Build) beta (64-bit)
Revision	18977fb43f693d5a8deeb48bd1cfa52651d3f73e
OS	macOS

The custom notification ad was not shown at all. The unblinded token for view was redeemed
Probably due to #18799