Fix errors in WallpaperManagerService

In a few cases, engines were not properly attached/detached. Also fix a
bug when loading settings (the new version creates new WallpaperData
objects instead of reusing old ones, which caused a bug)

This CL should change no logic when the mEnableSeparateLockScreenEngine flag
is false.

Bug: 253507223
Test: atest WallpaperManagerTest#testEngineCallbackCounts
Change-Id: I7094890aae7b3e204d5184476070b68a3cfe569d