…component is reattached into a recreated parent shell

When a stored component (e.g. EventListCmp at path '') is reattached into
a fresh parent shell (one that is destroyed and recreated on every
navigation because it is not covered by the reuse strategy), child routes
deeper than the stored component could fail to render.

The root cause was in `ChildrenOutletContexts.onOutletDeactivated()`. It
replaced `this.contexts` with a new empty Map and returned the old one for
storage. When the shell was recreated, its fresh router-outlet created a
brand-new OutletContext for the stored component, and `onOutletReAttached()`
loaded the saved contexts into *that* new context's `.children`. However,
the router-outlet *inside* the stored component (which stayed alive while
detached) was injected with the *original* `ChildrenOutletContexts` at
component creation time and held that reference for its lifetime. Because
the original instance's Map had been replaced with an empty one, subsequent
`activateWith()` calls on that outlet created grandchild components with a
fresh, empty context. Those grandchildren's outlets registered themselves
there, but the router traversed the *restored* context tree (a different
object) and never found an outlet to activate into — so the grandchild
component was silently dropped.

Fix: keep the same Map instance in `onOutletDeactivated()` rather than
swapping it out. This ensures that any outlet injected with the original
`ChildrenOutletContexts` and the router's restored context tree both see
the same backing Map after reattach. Stale outlet references left behind by
destroyed components are already cleaned up naturally through
`ngOnDestroy` → `onChildOutletDestroyed()`.

Fixes angular#57285