Add python script that generates pngs from svgs

- Requires rsvg-convert and the python sh module
- Has svgs in subfolders of icon-svgs/, where the name of the
  subfolder is taken to be the size of the (square) icon in dp
- Also add ldpi support. 8% of devices, and it is trivial for us to
  support anyway
- Maintains aspect ratio by fixing height

Change-Id: Icd9a8fbc9ff698036c28f53060c120ebe46165af