Revert CentralNotice banner box-sizing to content-box on mobile

The CSS for CentralNotice banners is typically the same on desktop
and mobile, and generally uses content-box box-sizing (which is the
default).

Change-Id: I7baf871ae05bedce4ebebadc58a4d57c4f72e39d
Merge "Revert CentralNotice banner box-sizing to content-box on mobile"