fix(material/tree): Apply aria-level to all nodes

Previously, only leaf nodes had aria-level applied.

This is an incremental change since this is an unfamiliar codebase for
me. The main benefit it will have on its own is that it will allow
anyone doing custom dom manipulation to know what level the node is on.
Otherwise by itself there is no change in how NVDA reads nodes with
children. (It currently reads them as literally "grouping"; no
information about the contents is provided).

This change will be necessary for a later change I'm planning, wherein
the role of parent nodes will be changed from "group" to "treeitem", in
accordance with how roles are applied in WAI-ARIA reference examples
such as
https://www.w3.org/TR/wai-aria-practices/examples/treeview/treeview-1/treeview-1b.html

Author: martiansnoop

src/cdk/tree/tree.ts

@@ -291,7 +291,7 @@ export class CdkTree<T> implements AfterContentChecked, CollectionViewer, OnDest
   exportAs: 'cdkTreeNode',
   host: {
     '[attr.aria-expanded]': 'isExpanded',
-    '[attr.aria-level]': 'role === "treeitem" ? level : null',
+    '[attr.aria-level]': 'level',
     '[attr.role]': 'role',
     'class': 'cdk-tree-node',
   },

src/material/tree/node.ts

@@ -44,7 +44,7 @@ const _MatTreeNodeMixinBase: HasTabIndexCtor & CanDisableCtor & typeof CdkTreeNo
   inputs: ['disabled', 'tabIndex'],
   host: {
     '[attr.aria-expanded]': 'isExpanded',
-    '[attr.aria-level]': 'role === "treeitem" ? level : null',
+    '[attr.aria-level]': 'level',
     '[attr.role]': 'role',
     'class': 'mat-tree-node'
   },