Remove background from unpinned headers

When ContactEntryListAdapter headers are unpinned, there is no
need to set their background color. Set the backgound to null.

Since the background color that is currently set on ContactEntryListAdapter
headers is currently incorrect in some unpinned contexts, this is an easy
solution to the mismatched background color.

Bug: 16552544
Change-Id: Ie17dfa40b495d87c6664605bd77e703acfdd2855
1 file changed