[Android][Shell] BackButtonBehavior IconOverride with FontImageSource renders stretched navigation icon

### Description

- When Shell navigation/back icon is set via BackButtonBehavior.IconOverride using FontImageSource on Android, the icon renders stretched/squished.

| Android | iOS |
|----------|----------|
| | |

### Steps to Reproduce

1. Deploy [ManualMaui](https://github.com/dotnet/maui/tree/main/src/Controls/tests/ManualTests) to Android.
2. Open: G Commands > G4 (Shell BackButtonBehavior binding Command to valid ICommand causes back button to disappear).
3. Observe: the back icon in the top-left is visible but appears stretched when IconOverride uses FontImageSource.

### Link to public reproduction project repository

https://github.com/dotnet/maui/tree/main/src/Controls/tests/ManualTests

### Version with bug

10.0.70-ci.inflight.26251.7

### Is this a regression from previous behavior?

No, this is something new

### Last version that worked well

_No response_

### Affected platforms

Android

### Affected platform versions

_No response_

### Did you find any workaround?

_No response_

### Relevant log output

```shell

```

Deploy the ManualMaui reproduction from src/Controls/tests/ManualTests, then open G Commands > G4 and inspect the Shell BackButtonBehavior IconOverride path using FontImageSource on Android. Reproduce the stretched back icon and trace the Android rendering/layout entry point; done means the navigation icon keeps its correct proportions, with the existing iOS behavior still intact.

Written by the indexing model from the issue text.