[ADD14-ish] Fix accent colours

Fix all the accent colours in drawables to match the app's main accent 
colour. Rename @color/accent_1 to @color/accent, and create the
@color/accent_list_bg for pre-4.0 list highlight BGs. Finally, re-crunch
all the PNGs using ImageOptim to shave off ~95 KB.

We might also consider inserting some branding guide with a checklist
of changes that have to be made when using iosched for a new
conference.