Account chips in the gr-account-list and their child elements existed in
the tabindex needlessly. These elements must remain focusable for a11y
reasons, but setting the tabindex to -1 allows for quick tabbing from
the reviewers entry to the ccs entry.
Bug: Issue 5277
Change-Id: Ifc3c4c5200de4fa0476493fd6ab6dabe7d5bdbe0