Be more lenient when processing group headers

We are now only looking at the contracted header instead of all
sorts of headers and if the current header is null, we don't
look any further but consider it to be equal.

Bug: 62378776
Test: add notification groups with multiple headers, observe correct icon / text behavior
Change-Id: I0b83b91caaace30df3bdcba7225e93bd0e195484