Reload task list when task is removed

Instead of individually removing tasks,
ask for all tasks to get accurate list.
This method is invoked whenever user presses
back on a root activity, which causes the task to
be killed from perspective of the activity.

Test: Visually inspected, recent task no longer
disappears.
Open any app, hit back, and then go to overview quickly.
App should remain in recents list.

Fixes: 135687618
Change-Id: I1c135673ae987016db5df0b83f5ea8e345d3c7c1
(cherry picked from commit 8651219f)