MaterialTextField setting background to Transparent not working

## 🐛 Bug Report

When setting the `BackgroundColor` to `Transparent` on a `MaterialTextField` the `BackgroundColor ` is still filled with the default color, ignoring the `Transparent `property on Android. This is tested with a `MaterialTextField ` inside a `Grid ` with a Background image on Android. The Background image is a `` component with a `rowspan ` of the total grid rows, and Aspect set to `AspectFill`. It is not (yet) tested on iOS.

I also tested it without the background image and it results in the same (the Transparent property being ignored).

### Expected behavior
The Filled color should be transparent of the MaterialTextField and not default back to the `#DCDCDC` value.

### Screenshots

### XAML Code
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### Configuration

**Version:** 1.7.4

**Platform:**
- [ ] :iphone: iOS
- [x] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [x] :monkey: Xamarin.Forms

### Additional information

- Using Xamarin.Forms 4.8.0.1364
- Using Xamarin.Essentials 1.5.3.2
- Targeting framework: Android 10.0 (API Level 29 - Q)

**Additional used libraries:**
- Splat 9.5.37