Use DesktopExperienceFlags for desktop experience wallpaper gating

This enables user override via the desktop experience developer settings.

Test: atest SystemUITests:GradientColorWallpaperTest
Test: atest FrameworksMockingServicesTests:WallpaperManagerServiceTests
Test: run go/cd-smoke
Test: disable cd wallpaper flag via adb and then enable the desktop experience developer option and verify the desktop wallpaper experience is enabled.
Flag: android.app.enable_connected_displays_wallpaper
Flag: com.android.window.flags.show_desktop_experience_dev_option
Bug: 393372870
Change-Id: I82c26b728626ff27e2c2475d4b603857f0b27fa2