Clarify semantics of changingToSame function

WallpaperManagerService#changingToSame takes both the "new" wallpaper
component and the "new" WallpaperData to decide if the new live
wallpaper is the same as the old. But it only used two fields from
WallpaperData, both of which still referred to the current wallpaper.
This change makes the "new" versus "current" semantics clear.

Bug: 347235611
Test: switching live wallpapers works as expected
Flag: EXEMPT cleanup
Change-Id: Ifb9198337a3dfade196d0e18423c2050433bebb4