WindowManagerService: Fix IndexOutOfBoundsException when traversing AppFreezeListener

Suggested by: Riddle Hsu <riddlehsu@google.com>

Change-Id: I4573d68568e3293074efa66914880385fc2afbc4
Signed-off-by: minaripenguin <minaripenguin@users.noreply.github.com>
Signed-off-by: Anushek Prasal <anushekprasal@gmail.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>