Clear the right target upon wallpaper failure.

FLAG_SYSTEM is simply wrong. We want to clear the destination where the
wallpaper crashed.

Failing to do so can lead to hard-to-recover states.

Flag: EXEMPT bugfix
Bug: 412664991
Test: atest WallpaperManagerTest
Change-Id: Ibf72d81a55525623c5c588a2e5de0e1791911ebc