You can try polygon overlay with holes in the middle. i.e. your entire map is covered by a polygon with an opacity value less then one. You can specify holes in the middle of the polygon. See the documents of polygon overlay:
http://developer.android.com/reference/com/google/android/gms/maps/model/Polygon.html
It says:
A polygon can be convex or concave, it may span the 180 meridian and it can have holes that are not filled in