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

Riproduci il problema nella Material datepicker examples URL aprendo example 1 e navigando fino alla data selezionata con le frecce della tastiera. Esamina l’implementazione del date range picker e il relativo focus styling; il lavoro è completato quando la data selezionata ha un focus indicator visibile oltre al proprio colore di sfondo.

Scritto dal modello di indicizzazione a partire dal testo della issue.