Update active time to visible leaf tasks

Update active time of leaf tasks that is visible. And make sure the
focused leaf task has the latest active time among all visible leaf
tasks.

Bug: 207185041
Test: atest RunningTasksTest
Test: trigger multiple split pairs, verified there's no crash when
      switching between split pairs
Change-Id: Ifbc6feef4d7810af92a7e354a9ba81804e7ec8da