Fix the leak while remove the taskFragment

ConcurrentModificationException is thrown from the ForEach function
while dealing with the dead process.  Task vanished event is sent
when remove task fragments and it updates the organized taskFragments
list at the same time.

Bug: 194728551
Test: - Launch activity to split and adb shell am force-stop process
Change-Id: I3a888def3337417565178407f13019f751bca017