Cleanup KeepAndroidOpen dialog code

[Stypox]

What is it?

• Bugfix (user facing)
• Feature (user facing) ⚠️ Your PR must target the refactor branch
• Codebase improvement (dev facing)
• Meta improvement to the project (dev facing)

Description of the changes in your PR

Adds a popup as discussed in #13438. Also cleans up the KeepAndroidOpen code a bit. This targets master because after this is merged a new release should be done, v0.28.6. I already wrote a blogpost for it, TeamNewPipe/website#434.

Before/After Screenshots/Screen Record

Fixes the following issue(s)

• Fixes Drop support for Android 5 / Require Android 6 / API 23 #13438

APK testing

The APK can be found by going to the "Checks" tab below the title. On the left pane, click on "CI", scroll down to "artifacts" and click "app" to download the zip file which contains the debug APK of this PR. You can find more info and a video demonstration on this wiki page.

Due diligence

• I read the contribution guidelines.
• The proposed changes follow the AI policy.
• I tested the changes using an emulator or a physical device.