Load account type icons in a background thread before scrolling down

This reduces the jank when scrolling down in top-level Settings if you
happen to have several account types like Google, Twitter, Facebook, etc.

In the process of doing this, discovered that headers get built even when
they're not showing (SubSettings). Skip setting up the headers, which can
be expensive when there are several accounts.

Bug: 11006601
Change-Id: I8b4eafdcc2fc2beecac01317c3c8f07a82febcc0