RadioButton ellipse colors are not applied

### Description

Supposedly you should be able to configure the colors of the RadioButton ellipses using Brushes with [these keys](https://github.com/dotnet/maui/blob/b0ea772fff1466e9d6b8d8660d4ffce03c202b96/src/Controls/src/Core/RadioButton/RadioButton.cs#L54-L64) according to [this PR](https://github.com/dotnet/maui/pull/14117).

However, this doesn't work as expected.

First issue I think I could identify is that [`Application.Current.TryGetResource`](https://github.com/dotnet/maui/blob/main/src/Controls/src/Core/ResourcesExtensions.cs#L58) as used in [`RadioButton.BuildDefaultTemplate`](https://github.com/dotnet/maui/blob/b0ea772fff1466e9d6b8d8660d4ffce03c202b96/src/Controls/src/Core/RadioButton/RadioButton.cs#L502) via [`BindableObjectExtensions.TrySetDynamicThemeColor`](https://github.com/dotnet/maui/blob/main/src/Controls/src/Core/BindableObjectExtensions.cs#L168) and [`BindableObjectExtensions.TrySetAppTheme`](https://github.com/dotnet/maui/blob/main/src/Controls/src/Core/BindableObjectExtensions.cs#L195) doesn't seem to find those brushes in merged dictionaries - only if they are in the top level `Application.Resources`.
So I'd have to customize these colors in my `App.xaml` instead of my `Colors.xaml`, where I would expect to do it.

But even if I define brushes with the magic keys in my `App.xaml` like the following, they're not applied.

```xaml
#954C2E
#954C2E
#954C2E
#954C2E
#954C2E
#954C2E
```

### Steps to Reproduce

_No response_

### Link to public reproduction project repository

_No response_

### Version with bug

10.0.70

### Is this a regression from previous behavior?

Not sure, did not test other versions

### Last version that worked well

Unknown/Other

### Affected platforms

Android, Windows

### Affected platform versions

_No response_

### Did you find any workaround?

_No response_

### Relevant log output

```shell

```

Start with RadioButton.BuildDefaultTemplate in src/Controls/src/Core/RadioButton/RadioButton.cs, then trace TrySetDynamicThemeColor and TrySetAppTheme in BindableObjectExtensions.cs and TryGetResource in ResourcesExtensions.cs. Reproduce the issue with the six documented brush keys in App.xaml and a merged Colors.xaml; done means the configured light and dark brushes are applied to the RadioButton ellipses from both resource locations.

Written by the indexing model from the issue text.