fix(cdk/tree): capturing focus on load (angular#29641)

The tree implements a roving tabindex which needs to have an initial item with `tabindex = 0` to work correctly. This happens by waiting for the data to be initialized in the `TreeKeyManager` and focusing the active/first item. The problem is that this ends up stealing focus on load. We didn't notice this issue in the demo app, because all the tree are `visibility: hidden` since they're inside closed `mat-expansion-panel`, but the issue is visible in the docs site.

These changes resolve the issue by setting the `tabindex` without actually moving focus.

Fixes angular#29628

Author: crisbeto

src/material/select/select.html

@@ -44,6 +44,7 @@
   (detach)="close()">
   <div
     #panel
+    cdkFocusable
     role="listbox"
     tabindex="-1"
     class="mat-mdc-select-panel mdc-menu-surface mdc-menu-surface--open {{ _getPanelTheme() }}"