Show changelog after installing update

[cl-ement05]

Please check all boxes below by replacing space with x

• [X] I am using the latest version
• [X] This is a suggestion for in the Vanced Manager and ONLY Vanced Manager (NOT Youtube Vanced/Music/microG)

Suggestion: Very simple : when Vanced Manager has been updated, show the changelog

Why is this suggestion relevant? Because it is interesting and good to know what changed were made

Further details: So I have an idea of how could this feature be implemented :

1. Add a shared preferences int value like "PreviousVersionCode" which, like its name suggests, contains value of previous version code
2. When an update is installed, do not change this value
3. So that on next app boot, VERSION_CODE constant will be different from PrevVersionCode
4. This way, we can know for sure the app was updated and show the changelog
5. After the chagelog has been displayed, change PrevVersionCode to VERSION_CODE to avoid showing change log multiple times

I know this way of doing is not very optimized but this is the only way we can now for sure Vanced Manager was updated. Because as it was pointed out by X1nto, we cannot retrieve Package Manager installation exit code on each ROM :

I guess it's possible to implement a callback which checks app status after it's uninstalled but I'm afraid it might not work equally on all devices and ROMs