[MaterialButtonToggleGroup] No check icon displayed to indicate selected state

Nobody has claimed this yet.

**Description:**
On the [M3 components website](https://m3.material.io/components/segmented-buttons/overview), it is stated that a check icon will be displayed to indicate selected state.

However in the [docs](https://github.com/material-components/material-components-android/blob/master/docs/components/Button.md#toggle-button) there's no mention of this. There is the `app:icon` property which sets the icon, but doesn't display the check icon when a button is selected in MaterialButtonToggleGroup.

**Expected behavior:**
| Expected behavior | Current behavior |
| -- | -- |
| | |

**Source code:**
```

```

**Android API version:**
34 (and very likely any other version too)

**Material Library version:**
1.12.0
1.13.0-alpha-06
1.13.0-alpha-05
1.13.0-alpha-04
1.13.0-alpha-03

Start with the MaterialButtonToggleGroup and MaterialButton implementation related to checked state and icons, using the XML example in this issue to reproduce the behavior. Compare the selected-state rendering with the M3 segmented-button reference and existing component documentation. Done means the selected button displays the expected check icon without breaking the configured icon behavior, with coverage or documentation updated as appropriate.

Written by the indexing model from the issue text.