Move background blur to Dim layer

The way background blur is expected to work is the same as the dim
layer - it should be cropped to the task/displayArea bounds and dim
layers shouldn't stack on top of each other. This CL makes the
background blur benefit from that and sets it on the dim surface
control.

Bug: 171679369
Test: m && flash
Test: atest DimmerTests

Change-Id: If64cc585a8a74ab5f3dc0757e25556fd89b5eaec