bug(MatInput): read-only inputs are not considered focused

#### Reproduction
[StackBlitz](https://stackblitz.com/edit/focused-readonly-input-bug)

Steps to reproduce:
1. Use the toggle to place the input in a read-only state
2. Tab to or click on the input to give it focus
3. Observe that the input has focus, but `MatInput` does not report it as focused


#### Expected Behavior
According to the Material [Text fields spec -> Read-only fields](https://material.io/components/text-fields#input-types)
>A read-only text field is styled the same as a regular text field.

Read-only inputs should receive the same focus styles as regular inputs. The `MatFormField` component conditionally applies focus styles (`mat-focused` class) based on the `focused` property of the `MatInput`.
https://github.com/angular/components/blob/master/src/material/form-field/form-field.ts#L139

When a read-only input is focused, `MatInput` should report it as focused to ensure focus styles are applied correctly.

#### Actual Behavior
`MatInput` filters out focus events on read-only inputs, always reporting them as not focused.
https://github.com/angular/components/blob/master/src/material/input/input.ts#L344

This results in the `mat-focused` class never being applied to `MatFormField`'s that wrap read-only inputs.

#### Environment

- Angular: 10.2.3
- CDK/Material: 10.2.7
- Browser(s): N/A
- Operating System (e.g. Windows, macOS, Ubuntu): N/A

リンク先の StackBlitz を使って動作を再現し、その後 src/material/input/input.ts の344行目付近と src/material/form-field/form-field.ts の139行目付近を調べてください。読み取り専用の input にフォーカスしたことが報告され、周囲の MatFormField に mat-focused クラスと対応するフォーカススタイルが適用されることを確認してください。

索引モデルが issue の本文から書いたものです。