fix(cdk/tree): refactor rendering pipeline

This also refactors the parent/level/group data processing in order to
make it significantly more consistent in all tree configurations.

Author: BobobUnicorn

src/cdk/a11y/key-manager/tree-key-manager.ts

@@ -360,8 +360,11 @@ export class TreeKeyManager<T extends TreeKeyManagerItem> {
   }
 
   private _updateActiveItemIndex(newItems: T[]) {
-    if (this._activeItem) {
-      const newIndex = newItems.indexOf(this._activeItem);
+    const activeItem = this._activeItem;
+    if (activeItem) {
+      const newIndex = newItems.findIndex(
+        item => this._trackByFn(item) === this._trackByFn(activeItem),
+      );
 
       if (newIndex > -1 && newIndex !== this._activeItemIndex) {
         this._activeItemIndex = newIndex;

src/cdk/tree/tree-errors.ts

@@ -46,15 +46,3 @@ export function getTreeControlMissingError() {
 export function getMultipleTreeControlsError() {
   return Error(`More than one of tree control, levelAccessor, or childrenAccessor were provided.`);
 }
-
-/**
- * Returns an error to be thrown when a node is attempted to be expanded or collapsed when
- * it's not expandable.
- * @docs-private
- */
-export function getNodeNotExpandableError() {
-  return Error(
-    `The node that was attempted to be expanded or collapsed is not expandable; you might ` +
-      `need to provide 'isExpandable'.`,
-  );
-}