XF.UWP sample AutoPlay property not working on MainPage

## 🐛 Bug Report

Expanding on #312

Since I am interested in the first scenario "run from asset or bundle" with Autoplay I went a little further with XF.UWP sample and noticed another thing.

When I added the AutoPlay="True" and RepeatMode="Infinite" properties to the AnimationView on the AssetPage, it started playing without hitting the play button as expected.
But when i copied the animation view over to the MainPage, the AutoPlay property was not having any effect. The animation was loaded but did not play automatically.
Even weirder, after navigating to the AssetPage and back again, the MainView Animation started to play.
Something in the naviation, or the playing of the other animation, must have triggered it.
I looked for some kind of initialisation logic, but couldn't find any.

### Expected behavior
animationview AutoPlay property also works on MainPage
### Reproduction steps

### Configuration
Xamarin.Forms 4.8.0.1821
Visual Studio 16.8.3

**Version:** 4.0.8

**Platform:**
- [ ] :iphone: iOS
- [ ] :robot: Android
- [ ] :checkered_flag: WPF
- [x] :earth_americas: UWP (In case you mean XF.UWP with this flag, maybe not?)
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [x] :monkey: Xamarin.Forms

Start with the XF.UWP sample's AssetPage and MainPage, comparing how the AnimationView is initialized with AutoPlay="True", RepeatMode="Infinite", Animation="LottieLogo1.json", and AnimationSource="AssetOrBundle". Reproduce the behavior on MainPage, then navigate to AssetPage and back to confirm the difference. Done means the MainPage animation starts automatically without requiring navigation or a play-button action.

Written by the indexing model from the issue text.