Keep playlist headers visible while scrolling long playlists

[kenneySiu]

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

• Changes to fragment_playlist.xml to use a LinearLayout with a
  dedicated playlist_header_container slot above the RecyclerView
• Changes to PlaylistFragment.java to bind the header directly from
  the layout and return null from getListHeaderSupplier()

Before/After Screenshots/Screen Record

• Before:

screen-newpipe.mp4

• After:

screen-playlist-buttons.mp4

Fixes the following issue(s)

• Fixes Keep playlist control buttons while scrolling long playlist lists #13502

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.