Set the wallpaper connection when migrating from system to lock wallpaper

Problem:
The `WPMS#updateFallbackConnection` function isn't reliably updating fallback wallpapers on all displays. This is because when a static home/lock wallpaper changes to a static lock-only wallpaper, the `mLastLockWallpaper` connection isn't properly linked during the initial update in `migrateStaticSystemToLockWallpaperLocked`, leading to an inconsistent state before the `WallpaperDestinationChangeHandler#complete` links the connection.

Solution:
Set the lock wallpaper connection in `migrateStaticSystemToLockWallpaperLocked`

Test: atest FrameworksMockingServicesTests:WallpaperManagerServiceTests
Flag: EXEMPTED small bug fix
Fix: 420579203
Change-Id: I87dd0fbd0e67301b44e0b78e3da72dc7fa075ffd