fix(cdk/tree): misc focus management fixes (#28136)

Fix miscelaneous focus management details in response to PR feedback.

 * omit tabindex attribute on role="tree" element
 * Remove TreeKeyManagerStrategy#onInitialFocus
 * Implement focus/unfocus pattern for TreeKeyManagerOption and use a
   binding to set tabindex on tree nodes
 * algin with https://www.w3.org/WAI/ARIA/apg/practices/keyboard-interface/#focusabilityofdisabledcontrols
 * Add tree-using-legacy-key-manager.spec.ts

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

@@ -50,10 +50,6 @@ export class NoopTreeKeyManager<T extends TreeKeyManagerItem> implements TreeKey
     return null;
   }
 
-  onInitialFocus() {
-    // noop
-  }
-
   focusItem() {
     // noop
   }

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

@@ -39,6 +39,11 @@ export interface TreeKeyManagerItem {
    * Focuses the item. This should provide some indication to the user that this item is focused.
    */
   focus(): void;
+
+  /**
+   * Unfocus the item. This should remove the focus state.
+   */
+  unfocus(): void;
 }
 
 /**
@@ -100,14 +105,6 @@ export interface TreeKeyManagerStrategy<T extends TreeKeyManagerItem> {
   /** The currently active item. */
   getActiveItem(): T | null;
 
-  /**
-   * Called the first time the Tree component is focused. This method will only be called once over
-   * the lifetime of the Tree component.
-   *
-   * Intended to be used to focus the first item in the tree.
-   */
-  onInitialFocus(): void;
-
   /**
    * Focus the provided item by index.
    *