Look for a new wallpaper target if ours went away

This is necessary when we're closing something over a non-activity
window, eg. lock screen / notification shade.

Bug: 293862576
Test: atest WallpaperServiceTest
Test: atest KeyguardTests
Change-Id: I82e153a56a6b28f06bd4af6e542c62e6473dca10
1 file changed