Make sure cleanup is always done when task is removed

When Stack#removeImmediately() was called it also recursively
called Task#removeImmediately(), which remove tasks from stack.
This lead to Task#mStack reference being nullified, but task
dim layer user was still registered in DimLayerController.
Therefore there was a crash when dim layer animation occured.

This CL moves most of the logic from Task#removeIfPossible() to
Task#removeImmediately() to make sure that cleanup is performed
every time when task is removed.

Change-Id: Id8d72dc8c66b9eeefbf5c918cf0a0df4ea027fde
Fixes: 34052466
Test: bit FrameworksServicesTests:com.android.server.wm.TaskStackTests
Test: #testStackRemoveImmediately