Fix bg blur not showing when layer alpha is 0

A layer should be considered visible when the layer has a background
blur value. The blur is always rendered at full opacity, even if the
layer alpha is 0.
This CL makes an effect layer visible when the layer has background
blur.

Bug: 167166562
Test: atest EffectLayerTest#BlurEffectLayerIsVisible
Change-Id: I8d2a0988b185b1caef4fc458dff7ec4e8976ae16