Don't send color events when dim doesn't change

Things (battery saver in our case) sometimes call setWallpaperDimAmount
and the call doesn't affect the dim. In that case, skip recomputing
colors and sending color events.

This mostly deflakes WallpaperManagerTest, but may spare useless
computations in some cases.

Flag: NA
Bug: 326189127
Test: atest WallpaperManagerTest
Change-Id: I5e101d012b18420bfa5b579db78b33300bfb1659