Site picker: update behavior to better match other platforms

[mattmiklic]

Our site picker on iOS differs from Android and the web in some small but important ways. I'm creating this issue to track the changes we should make on iOS to better align its picker with other platforms.

In Calypso, if the user has 11 or more visible sites, we show the 5 most recently-accessed sites on top of the list. On iOS, we only show 3 recent sites, and the threshold appears to be based on the user's total number of sites, not just what's visible. In Calypso, we show a border to divide the list of 5 recent sites from the rest of the list. On iOS, there is no divider.

• [ ] Only show recently accessed sites at top of site picker if 10 or more are visible.
• [ ] Show 5 recent sites rather than 3.
• [ ] Add a 1px greyLighten20 horizontal line to separate most recent sites from rest of sites (Example)