When a radio button is both checked and disabled tabindex wouldn't step into the radio group at all

#### Reproduction

Use StackBlitz to reproduce your issue: https://stackblitz.com/edit/angular-material-radio-button-demo-wz3p6o?file=app%2Fapp.component.html

Steps to reproduce:
1. Have a radio button which is both checked and disabled
2. try to navigate to radio group through tab


#### Expected Behavior
If checked radio button is also disabled the tab should navigate you to the first enabled radio button (even if is not checked)

#### Actual Behavior
If checked radio button is also disabled radio group is not navigable through keyboard

- Angular:

从链接的 StackBlitz 复现开始，跟踪单选按钮组在选中且禁用的按钮上的键盘导航行为。完成标准是 Tab 导航能够到达第一个启用的单选按钮，而不是跳过整个组。

由索引模型根据 Issue 内容生成。