Custom ad notifications are not shown when system notifications are disabled

We should fall back to custom ad notifications when system notifications are disabled

Follow up to: https://github.com/brave/brave-variations/pull/120

Should show custom ad notifications:
- For all Windows versions If native notifications are disabled (should not be shown if Focus Assist is enabled)

Steps to Reproduce

1. Disable native(system) notifications
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 --brave-ads-staging --brave-ads-debug --rewards=staging=true --variations-server-url=https://variations.bravesoftware.com/seed
3. Restart Brave
4. Make sure variations are downloaded in brave://version
5. Enable rewards and ads
6. Trigger an ad

Actual result:

Notification is not shown at all

Expected result:

Custom notification is shown

Reproduces how often:

Easily reproduced

Brave version (brave://version info)

Brave	1.30.84 Chromium: 94.0.4606.54 (Official Build) (64-bit)
Revision	c8191a1d5cccbf64e8fe7269043f8ace8d74dd05-refs/branch-heads/4606@{#1130}
OS	Windows 10 Version 20H2 (Build 19042.1237)

cc @brave/legacy_qa @rebron @tmancey @jsecretan

[btlechowski]

Needs to be verified along with https://github.com/brave/brave-variations/pull/128

Verification should include all test cases from https://github.com/brave/brave-variations/pull/120

[GeetaSarvadnya]

Verification passed on Gionee s5 with Android 5.1 running Bravearm 1.30.89.apk build

1. Verified when system native notifications are disabled and variations seed enabled, custom ads notification is shown based on BraveAds.ShowCustomAdsNotificationOnAndroidStudy: Enabled being enabled

Steps followed:

1. Run the cmd mentioned in the screenshot below
2. Install bravearm.apk on Android 5
3. Disable native app notifications in Android 5 (navigate to Android 5>Advanced settings>Apps>Brave, uncheck Receive app notifications checkbox)
4. Check brave://version and ensured enable custom ad notification seed isn’t downloaded
5. Restart browser, check brave://version and ensured BraveAds.ShowCustomAdsNotificationOnAndroidStudy: Enabled seed is downloaded
6. Enable rewards and ads
7. Custom ads are served

Native app notification disabled	Custom ad study enabled	Custom ad notifications shown

2. Verified when system native notifications are enabled, native ad notifications are shown

Steps followed:

1. Run the cmd mentioned in the screenshot below [Note: make sure to run adb.exe shell "rm /data/local/tmp/brave-test-variations-server-url" otherwise it will pull the variation seed from the server and you will get always custom ads even though native app notifications are enabled]
2. Install bravearm.apk on Android 5
3. Make sure native app notifications are enabled in Android 5
4. Check brave://version and ensured enable custom ad notification seed isn’t downloaded
5. Restart the browser, check brave://version and ensure enable custom ad notification seed isn’t downloaded
6. Enable rewards and ads
7. Default ads are served

Native app notification enabled	Custom ad study NOT enabled	Default ad notifications shown