Enable Custom Notification Ads on release channel for OSes which do not support native notifications

[btlechowski]

Currently ads will not show prior to Windows 10 version 1803, build 17134 and Mac prior to 10.14

We need to enable custom ad notifications so that user start receiving ads again.

Steps:

1. Clean profile
2. Run Brave
3. Wait for variations to be downloaded
4. Restart Brave
5. Enable rewards and ads
6. Trigger an ad

Expected behavior:
Custom Notification Ad is shown
CustomAdNotifications -> can_fallback_to_custom_notifications is enabled

cc @tmancey

[tmancey]

@btlechowski speaking with @jsecretan we have decided to also enable custom ad notifications for the following scenarios:

Windows 10 build 16299 and older
Windows 10 build 17134 and newer (fallback if native notifications are disabled, should honor Focus Assist)
macOS 10.13 and older
Android 7 and older
All Linux versions