Fix scrolling in blocked numbers list.

+ Add elements as header, so they scroll with the list.
+ Shuffle around some layouts to make this work correctly.
+ Replace default ListView divider with custom drawable, to achieve
the desired effect of removing the divider on the first list item.
+ Override BlockedNumberAdapter to never report itself as empty.
Otherwise, the header will disappear.
+ Tweak some padding.

Bug: 25134186
Change-Id: I07711437fea2d3042d3987983667ab5316d51fcc