Added limit of descendant amount when adding activity to task

This avoid bad apps to consume available layers of surfaceflinger.

The tree weight is a simple field (instead of iterating activities)
to indicate the total amount of descendant window containers.

Background launch restriction only disallow launching new task from
background. If the target task is an existing own task, the new
activity will still be added.

When exceeding this limit, the target task will be removed, so if
a problematical loop is still starting activity, it will becomes
a new task that will be blocked by background launch restriction.

Bug: 268590786
Bug: 270477256
Test: Endless loop to start activities in its own task in
      background. There won't have Surface$OutOfResourcesException.
Change-Id: Ief0d23477bd095f7e60a0a9079b66b3bc88f6df9