Revert "Replace Repeater with ListView for task group list"

This reverts commit ea84e5c1.

This commit was supposed to fix 433907 but apparently did not do so.
Additionally, it introduced an unpleasant visual flicker for grouped
task tooltips. Let's try again another way.

CCBUG: 433907
(cherry picked from commit 4342e6f7eb7d2eb7eb789554a8be2e5d1a8b618d)