Fix wallpaper dim logic

See b/318656512#comment11 for explanation and details about the change

This change is approved by the currently ignored tests:
WallpaperManagerTest#setDimAmount_lockScreenUnset_notifiesColorsChangedBothTogether
WallpaperManagerTest#setDimAmount_lockScreenSet_notifiesColorsChangedBothSeparately

But they should still be @ignored for now due to an unrelated flake
(SecurityException thrown occasionally).

Flag: NA
Bug: 318656512
Test: manual with logs
Test: atest WallpaperServiceTest, WallpaperServiceTests
Test: atest WallpaperManagerTest (the two setDimAmount_ tests)
Change-Id: I92f66123ce0643dc8115d92a40488ef5ba360bf0