Add an activity log

The inspiration for **App usage** was the apps such as Digital Wellbeing created by Google and Samsung. The original plan was to add some diagrams, graphs to make it more fancy like others have done. But it doesn't go well with the philosophy of AM. It needs more tuning and features in order become actually useful to the users. My present idea is to create a new activity tab which will list every events such as app usage (foreground/background), data usage, notifications (requires a background service, unfortunately), app ops usage sequentially (possibly the most recent events on top). There will be additional options for each of these types which can be used to modify the corresponding app events (when possible).