Use of custom TrackByFunction in MatTree does not preserve expansion state when nodes change

#### What is the expected behavior?

Use of trackBy should also ensure that the expansion state does not get lost when nodes are updated.

#### What is the current behavior?

Use of trackBy does not ensure that the expansion state does not get lost when nodes are updated. This is presumably since TreeControl uses an expansionModel which is a SelectionModel where the SelectionModel has no awareness of the TrackByFunction.

#### What are the steps to reproduce?

Relying upon the TreeControl expansionModel:
https://stackblitz.com/edit/angular-5wpr9o-kjnmsn?file=app%2Ftree-flat-overview-example.ts

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?

all

#### Is there anything else we should know?

Trying to store expansion state in the nodes themselves as a workaround doesn't work (unless you transfer the state):
https://stackblitz.com/edit/angular-5wpr9o-ahptjs?file=app%2Ftree-flat-overview-example.ts

Having a custom expansionModel that is aware of the trackBy state works to preserve state, since the TreeControl's expansion-related methods apparently aren't actually used internally by the tree component:
https://stackblitz.com/edit/angular-5wpr9o-pbmk1g?file=app%2Ftree-flat-overview-example.ts

Inizia dalle riproduzioni StackBlitz collegate, in particolare app/tree-flat-overview-example.ts, e analizza come interagiscono MatTree, TreeControl, TrackByFunction e expansionModel. Riproduci la perdita dello stato di espansione quando i nodi vengono aggiornati con un trackBy personalizzato. Il lavoro è completato quando lo stato di espansione rimane preservato dopo questi aggiornamenti.

Scritto dal modello di indicizzazione a partire dal testo della issue.