Fix unexpected closure of the bubble.

WindowManager calls ATMS#notifySingleTaskDisplayEmpty to notify that the
last task is removed from a single task display.

Originally it was called from ActivityStack when Tsak is removed, but
while merging ActivityStack and Task, CL[1] moved the call to Task. As a
result, the lister gets notification when ActivityRecord is removed,
instead of a last Task is removed, which causes unexpected bubble
closure.

[1]: Ifb167129c89a3aba11796daa21a9dee7200913ca

Bug: 157067689
Test: atest TaskStackChangedListnerTest
Change-Id: I93b745ba7fd1bb2fbef7915ddff2316100813187