"Notify when" notifications don't trigger

###### Reproduction Steps

1. Activate a Notify when X cards are due setting
2. Assume X cards are due
3. Notification does not show unless I manually start the app.

###### Expected Result

Notification to appear automatically, regardless of app state.

###### Actual Result
Notification only appears when app is started manually.

###### Debug info
Refer to the [support page](https://ankidroid.org/docs/help.html) if you are unsure where to get the "debug info".

###### Research
*Enter an [x] character to confirm the points below:*

- [x] I have read the [support page](https://ankidroid.org/docs/help.html) and am reporting a bug or enhancement request specific to AnkiDroid
- [x] I have checked the [manual](https://ankidroid.org/docs/manual.html) and the [FAQ](https://github.com/ankidroid/Anki-Android/wiki/FAQ) and could not find a solution to my issue
- [x] I have searched for similar existing issues here and on the user forum
- [ ] (Optional) I have confirmed the issue is not resolved in the latest alpha release ([instructions](https://docs.ankidroid.org/manual.html#betaTesting))