Strengthen Wallpaper clear logic

The changes are (all under flag):
 - support calling clearWallpaperLocked(FLAG_LOCK | FLAG_SYSTEM), which
   can be done more efficiently than clear(FLAG_LOCK) then
   clear(FLAG_SYSTEM).
 - add a check INTERACT_ACROSS_USER_FULL instead of returning if
clear is called for another userId than mCurrentUserId
 - use setWallpaperComponent and not bindWallpaperComponentLocked in
   clear, since setWallpaperComponent does other important things that
   we need when changing wallpapers. This is the most important change.

Also amend the javadoc to clarify the clear behaviour.

Flag: lock screen lwp
Bug: 283091821
Test: atest WallpaperManagerTest
Test: manual: disable security checks, then for all possible
combinations of static/live wallpaper(s), run:
"adb shell service call wallpaper 10 s16 "test" i32 X i32 0"
for X in (1, 2, 3) (resp. system, lock, system | lock)

Change-Id: I86a9f18a1d236633e4de3923861a5b55c7c4c996