Show group headers correctly

- have the list's top padding inside the first group header so
  that the list shows correct fading effect on top
- have a header divider at the bottom of the header itself
- fix text alignment so that it looks same as the "All contacts"
  list

Bug: 5233999
Change-Id: I2ea24b76cd2b2b3240ff3a9b705b7d783cacebee