Notify users about filter list initialization issues

[mario]

Background

It occasionally happens that iOS fails to reinitialize the filter list when it was deactivated and activated again in the iOS settings. iOS doesn't notify the user about the fact that the reinitialization failed, so we need to notify the user within our app.

Please note, that #3175 already introduces a workflow which asks the user to activate the ABP filter list in the iOS settings. This will be referenced in the "What to change" section.

What to change

In case ABP has already been initialized once (thus the first run slides are not active), as soon as ABP for iOS is opened check whether the filter list is loaded properly. If the filter list is not loaded properly, fall back to the filter list initialization process as introduced in #3175 (Step 2).

Please use the exact same steps to ask the user to initialize the filter list as in #3175:

1. ABP will display the notification that asks the user to initialize the filter list. (#3175 Step 2)
2. As soon as the filter list has been reinitialized, ABP will display a notification that the filter list is being processed. (#3175 Step 3)
3. As soon as the filter list has been processed, ABP will display a notification saying ABP is ready to use. (#3175 Step 4)

Remark for testers

You can make iOS (occasionally) fail at reinitializing the filter list by going into Settings -> Safari -> Content Blocking, deactivating the filter list, activating it again and then quickly closing the settings app.

[Mary31]

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