Fixes layout of icons in notification header.

This change resolves a discrepancy between onMeasure() and onLayout() in
NotificationHeaderView.  Icons were measured with their defined
margins in order to figure out how much to truncate the header text, but
during layout potentially multiple icons used the container's end
padding as their end margin (and their start margin was ignored).

Change-Id: I513efe2723fc78dd81bbe07aa40a3ed5fd1182d1
Fixes: 120893622
Test: manual