According to the developer preview of Android O which is to be launched at the end of this year, by defining a background and a foreground layer, developers will be able to use adaptive launcher icons. As the name suggests, these launcher icons can adapt depending on for example the device used. Now on one OEM device you could show round launcher icons and on another the square version. The OEM will provide a mask, which will allow for an unified overall look. According to android, the new launcher icons are used in shortcuts, the Settings app, sharing dialogs, and the overview screen.
As developer you only need to define the two layers and provide them in a PNG drawable format with the size of 108 by 108 dp. The inner 72 x 72 dp will be visible using a mask whilst the other 36dp will be used for a variety of animations provided by android. More details on how to create adaptive icons you can find on the Android developer website.
image source: developer.android.com