Revert "Remove "Droid Sans" from the font stack."

This reverts commit 1a24fa0baa01e48e6a706c1f2e31447020594165.
Looks like Android < 4 only has 3 fonts installed -
* Droid Sans
* Droid Serif
* Droid Sans Mono

So we do need to keep Droid Sans in.
http://www.granneman.com/webdev/coding/css/fonts-and-formatting/default-fonts/#android