Update icon files with new Material Design assets.

This patch does the following:

- Add a new android_icon.ico file for Windows, containing images
  for 14 different sizes!

- Add new PNG icon images for sizes 32, 128 and 256.

- Add a new script (generate-android-icons.sh) to automatically
  generate the content of android/icons.h, using the 'xxd' tool.

- Update android/icons.h with the new auto-generated content.

Change-Id: Iaa361f60f3e2081d73c604d3bd0aaa481250d89d
Merge "Update icon files with new Material Design assets."