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

Material datepicker examples URL で example 1 を開き、キーボードの矢印キーで選択された日付まで移動して問題を再現します。date range picker の実装とその focus styling を調査します。選択された日付に背景色に加えて視認できる focus indicator が表示されれば完了です。

索引モデルが issue の本文から書いたものです。