Fix WM missing stack/task when activity is force stopped and restarted

When activity process is force-stopped (like in the case we need to stop
it to allow debugger), activity record is removed and a new instance is
added. Depending on timing, adding of new record could happen before
or after old record is removed. If it added after old record is removed,
then the removal could cause AM to also remove the task AND the stack
(if the activity is the last one).

On WM side however, the task/stack removal could be delayed due to
animation. So when AM attaches a new stack, we end up with two stacks
with the same stack id in the DisplayContent. Also, AM reuses the last
used taskId after it's deleted. So WM is confused and didn't add a new
task record. Then when the delayed stack removal fires everything is gone.

We need to use a different task id when creating a new task record, even
if the last created task is already deleted. Also when attaching a new
stack to WM, first check if we already have such a stack in DisplayContent,
if so, reattach it and don't create a duplicate.

bug: 30465601
Change-Id: Ic399a32c54f6acdcbb9a0c6155599331d55df232