Fix 9patch padding for no/any dpi resourcesa

If a 9patch image was in the nodpi or anydpi folder,
the density of the image was 0 resulting in a float division by 0 when
layoutlib tries to scale the padding of the 9patch.

This fix set the density of the image to the same as the device when the
9patch comes from one of these folder.

Bug: 37136109
Test: Added a render test
Change-Id: If718659e6123f6d30bac8f2b35668093a6dbde43