New assets for checkboxes and radio buttons.

Added a dark/light variation.

Other files that might also need an update:
  btn_check_label_background_light.9.png
  btn_radio_label_background_light.9.png
  btn_check_label_background.9.png
  btn_radio_label_background.9.png

Pressed state is nearly invisible in light theme. To be tested with new background color.

Change-Id: Ibd312044364e08f1e789830ddd4c3af5cef30be1