fix(cdk/tree): add injectable key manager and opt-out

Make backwards compatibility improvements to cdk-tree-revamp regarding
focus management, the key manager and tabindex attribute.

 * Add TreeKeyMangerStrategy interface
 * Add injection toekn for tree key manager
 * Add LegacyTreeKeyManager

Provide LegacyTreeKeyManager to use legacy tabindex behavior from before
TreeKeyManager was introducted.

This commit message will be squashed away.

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

@@ -0,0 +1,91 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {Subject} from 'rxjs';
+import {
+  TREE_KEY_MANAGER,
+  TreeKeyManagerFactory,
+  TreeKeyManagerItem,
+  TreeKeyManagerStrategy,
+} from './tree-key-manager';
+
+/**
+ * @docs-private
+ *
+ * @deprecated LegacyTreeKeyManager deprecated. Use TreeKeyManager or inject a
+ * TreeKeyManagerStrategy instead. To be removed in a future version.
+ *
+ * @breaking-change 19.0.0
+ */
+// LegacyTreeKeyManager is a "noop" implementation of TreeKeyMangerStrategy. Methods are noops. Does
+// not emit to streams.
+//
+// Used for applications built before TreeKeyManager to opt-out of TreeKeyManager and revert to
+// legacy behavior.
+export class LegacyTreeKeyManager<T extends TreeKeyManagerItem>
+  implements TreeKeyManagerStrategy<T>
+{
+  get _isLegacyTreeKeyManager() {
+    return true;
+  }
+
+  // Provide change as required by TreeKeyManagerStrategy. LegacyTreeKeyManager is a "noop"
+  // implementation that does not emit to streams.
+  readonly change = new Subject<T | null>();
+
+  onKeydown() {
+    // noop
+  }
+
+  getActiveItemIndex() {
+    // Always return null. LegacyTreeKeyManager is a "noop" implementation that does not maintain
+    // the active item.
+    return null;
+  }
+
+  getActiveItem() {
+    // Always return null. LegacyTreeKeyManager is a "noop" implementation that does not maintain
+    // the active item.
+    return null;
+  }
+
+  onInitialFocus() {
+    // noop
+  }
+
+  focusItem() {
+    // noop
+  }
+}
+
+/**
+ * @docs-private
+ *
+ * @deprecated LegacyTreeKeyManager deprecated. Use TreeKeyManager or inject a
+ * TreeKeyManagerStrategy instead. To be removed in a future version.
+ *
+ * @breaking-change 19.0.0
+ */
+export function LEGACY_TREE_KEY_MANAGER_FACTORY<
+  T extends TreeKeyManagerItem,
+>(): TreeKeyManagerFactory<T> {
+  return () => new LegacyTreeKeyManager<T>();
+}
+
+/**
+ * @docs-private
+ *
+ * @deprecated LegacyTreeKeyManager deprecated. Use TreeKeyManager or inject a
+ * TreeKeyManagerStrategy instead. To be removed in a future version.
+ *
+ * @breaking-change 19.0.0
+ */
+export const LEGACY_TREE_KEY_MANAGER_FACTORY_PROVIDER = {
+  provide: TREE_KEY_MANAGER,
+  useFactory: LEGACY_TREE_KEY_MANAGER_FACTORY,
+};