DesktopMode: Make DesktopWallpaperActivity per-display.

This uses a combination of Intent flags - ACTIVITY_NEW_TASK and
FLAG_ACTIVITY_MULTIPLE_TASK and checking TokenProvider's current
existing token to ensure we aren't spawning new task containing
WallpaperActivity if they already exist.

Bug: 381935663
Test: atest and Manual checking on Connected Displays
Flag:com.android.window.flags.enable_per_display_desktop_wallpaper_activity

Change-Id: I95d4106a5e9194ece2931309c9908626c2060b22