Toggling IsVisible on MaterialCard will remove elevation in iOS

## 🐛 Bug Report

When you use a `MaterialCard` and have an `Elevation` value on it, then this elevation will disappear when toggling `IsVisible` to `false` and then back to `true` via `BindableProperties`. This only happens on iOS.

### Expected behavior
`MaterialCard` and `Elevation` will stay the same, even when hiding/showing multiple times.

### Reproduction steps
Step 1: Create a `Control` with a `MaterialCard` and put an `Elevation` value of '2' on it.
Step 2: Consume this `Control` in your `View`.
Step 3: Bind to the `IsVisible` property of that control within your `ViewModel` for that `View`.
Step 4: Toggle the `IsVisible` to `false` and then back to `true`.
Step 5: You should have seen the bug now!

### Configuration
- IPhone XR with iOS 14.0
- Xamarin Forms: v4.8.0.1687 (latest stable)
- Xamarin.Forms.Visual.Material: v4.8.0.1687 (latest stable)

**Version:** 1.7.4 (latest stable)

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