Refactor wallpaper activity token outside of repo.

 Refactor the reference to wallpaper token outside of repository to
  it's own class. This way we can consistently access the same task for different users.

Bug: 381396057
Test: atest DesktopTasksController
Test: atest DesktopTasksTransitionObserver
Flag: EXEMPT Refactor

Change-Id: I78ca1ce0a7759cd5ca813aac7895f6b701fc75be