Detach children when stopping app.

Prior to the implementation of detachChildren we handled this
case via the "mWindowStopped" codepath in SurfaceView.java which this
CL deletes. That codepath however causes confusion due to it's failure
to set null the SurfaceControl, meaning we may not necessarily
recreate it when resuming if we didn't hit any other code-path
to do such as happens in linked bug 78588930. Anyway it seems clearest
to handle all these preserve-child-surfaces-on-tear-down cases via
one mechanism (detachChildren).

Bug: 78588930
Test: Manual.
Change-Id: Iac7c0bc0c6b4da0d405bdc2b57d13d5c881611b0