If you really want to build against an SDK before xxhdpi was introduced, you can use drawable-480dpi. xxhdpi is just a synonym for that (like xhdpi is a synonym for 320dpi).+Ryan Chazen
SVG is really not such a clear benefit. Unless you have some kind of significant hinting in the image, rendering a vector graphic at say 240dpi is not going to give you much (if at all) of a different result than scaling down a higher density bitmap. The SVG rendering is still going to have many of the artifacts that people often think they avoid with vectors, such as grays along edges due to lines not being positioned on pixel boundaries.
I get a lot of pressure from people to have vector image support in Android resources, and my general feeling about this is that a prerequisite for that is to first have a build-side asset flow based entirely on vectors that real UI designers use as part of the bulk of their asset production, never touching bitmaps. And shows some significant advantage over simply generating the original assets as very high resolution bitmaps.
Also we are clearly reaching an upper limit on the density that we care about for bitmaps. If nothing else, I can't imagine most developers really caring about providing assets for any higher resolutions, and even xxhdpi I would say is only really of use for application icons just because of the pattern of those being used to provider larger icons for xhdpi devices.
(Btw allowing you to scale up images arbitrarily on the same density screen is much more likely to be a place where vector graphics would have some utility, rather than handling different screen densities.)