Filter shortcuts down to 4 if there are more.

- We take the first 4 in sorted order, except we remove up to 2
  static shortcuts to make room for dynamic ones if they exist.
- Added ShortcutFilterTest with testSortAndFilterShortcuts().
  This asserts that the filtered list is sorted and has the
  expected number of static and dynamic shortcuts based on
  inputs with various amounts of each.

Bug: 28980830
Change-Id: I832b6b21144f17c74bb8b90a840d6620e99911b8