Add font-family-system-sans font stack

This font stack is providing system specific fonts for improved
readability, especially on mobile devices.
It is meant to be used more widely and possibly replace
font-family-sans var in the mid-future.
We already use it in places like Design Style Guide.

Bug: T175877
Change-Id: I868e30ec3445f1226fa11a23f6d853b4acd0a728