add design tips to iconography page.
Accounts for the removal of the previous UI guideline pages,
which is where most of this new content came from.
Also update figure to add xxhdpi and remove ldpi,
and include 2x version, because ya know, irony.

Change-Id: Ia37ef1a65bdf9c4173a323ae37fffe03b53a6fcf
5 files changed