Restrict the cap # of frequents on home screen to 20

Bug: 10365481

- If the counter of starred contacts is less than 20, show maximum 20
  starred + frequents
- If the counter of starred contacts is no less than 20, show all the
  starred contacts only (no frequents)

Change-Id: If2b9533446671f37e64caffd85e8db330b857ecd