Android Studio Image Asset Launcher Icon Background Color
To make background transparent, set shape as None.See the image below:
EDIT:
For Android Studio 3.0,you can set it from Legacy Tab
I'm using Android Studio 3.0.1 and if the above answer doesn't work for you, try to change the icon type into Legacy and select Shape to None, the default one is Adaptive and Legacy.
Note: Some device has installed a launcher with automatically adding white background in icon, that's normal.
You have two ways:
1) In Background Layer > Scaling, reduce the Resize to 1
and then in Legacy > Legacy Icon set Shape as None
2) in Background Layer > Scaling > Source Asset, you can set an image as a 1x1 pixel (or any size) transparent.png image (you've already created).
and then in Legacy > Legacy Icon set Shape as None