Active container should contain non-finishing activity

Without this cl, the app will crash if a TaskFragment is closing when
the new top TaskFragment doesn't contain any non-finishing activity
(which should be closed later as well).

Before, when #cleanupContainer(), we will also update the new top active
container, but we didn't check if it contain any non finishing activity.

Fix: 212355346
Test: test with Settings with three finger click
Change-Id: I26538b85f1cf8e618a3f1ee6abada5a9f77f2b16