I found the following, hope this helps:
Android Icon Margins
… it was recommend that the icon included a margin around one twelfth of the total size between the edge of the icon and the image it contained. For example the medium density icon is 48×48 pixels, one twelfth of this is 4, giving a margin of 4 pixels around the icon. This means the area available for the image is 40×40 (48 minus the margin of 4 pixels on each side).
However, the bigger screens on today’s devices gives more room for the icons so a smaller margin can be used, for example one eighteenth of the icon size. The margin allows for spaces between the icons on a screen and to allow for any drop shadows or parts of the image that stick out a little.
http://tekeye.biz/2013/android-icon-size