Accessibility: make columnheader the primary role of the element

The role attribute overrides implicit roles. Since here the
columnheader role of the th is more important than the secondary role
of being a button, we should force it explicitly in that order.

Follow up of: I2b937eba605b7e6a98612a02c75eb9e7f3477527

Change-Id: I07211263876a58c62e5a349eab6f78e43d412680
Merge "Accessibility: make columnheader the primary role of the element"