android: add selection of included/excluded apps in split tunneling

[davfsa]

Hello!

I took a stab at adding the ability to select included/excluded apps from App Split Tunneling instead of only allowing selecting excluded applications.

Included apps view	Excluded apps view	Selected apps reset alert

All images are 1280x2856, so you can open them in a new tab to view them better

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A couple of points:

• DISALLOWED_APPS_KEY (now SELECTED_APPS_KEY) is in a bit of a weird situation as the value no longer represents the variable name. I havent opted to add a migration flow for the key, but can be done if needed. For now I have just added a comment above explaining the the reason
• switchUserSelectedPackages is not the best of names, but I couldn't come up with any better, will update with any suggestions or if I come up with a better one
• This is my first real work with Android JetPack and Android in general, so if there are any rookie mistakes, that would be why 🙃. If anything needs to be changed, I will get to it as soon as possible!

Closes tailscale/tailscale#14660