bug(mat-date-range-picker): Focus indicator is not present for the selected date

#### Reproduction

Steps to reproduce:
1. Open the URL https://material.angular.io/components/datepicker/examples
2. Open date range picker in example 1
3. Navigate to the selected date using keyboard arrow keys
4. Observe that when focus is on selected date , user cannot understand where the current focus is.


#### Expected Behavior

Provide a focus indicator or drop shadow in addition to the background color change

#### Actual Behavior

Focus indicator is not present for the selected state of date's present inside date range picker. Hence it is difficult for the users to know where the focus is

#### Environment

- Angular: 11.0.3
- CDK/Material: 11.0.3
- Browser(s): Chrome browser 90.0.4430.100
- Operating System (e.g. Windows, macOS, Ubuntu): Google Chrome OS

在 Material datepicker examples URL 中打开 example 1，并使用键盘箭头导航到所选日期，以重现该问题。检查 date range picker 的实现及其 focus styling；当所选日期除了背景色之外还具有可见的 focus indicator 时，即表示完成。

由索引模型根据 Issue 内容生成。