Calling stop function before the media is loaded doesn't work

## 🐛 Bug Report

When we play from a url, the mediamanager takes some time to load the video. if during this time we change the url, the mediaplayer continnues to load the previous url first and starts playing the first media and then plays the current one. So for the current label wrong video will be played at first and then the actual one. The first thing we might try to do to avoid this behaviour is to stop the media player before loading the next one. But this is also not working during that loading period.

### Expected behavior

Current item should stop playing when stop function is called even if it was still in loading state

### Reproduction steps

Create mediaplayer element in xamarin forms, play one video, before loading is complete call the stop function. It won't work, the media starts playing later.
Also play a url first. change the url before first media is loaded, The player first plays the first media and then loads the next one. The required behaviour would be to play the currently given url.

### Configuration

**Version:** 0.9.9 and below

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

Reproduce the issue in a Xamarin.Forms media player by calling stop while a URL is still loading, then by changing the URL before the first media finishes loading. Trace the media-loading and stop entry points to verify that the current item is cancelled and that only the currently assigned URL starts playing.

Written by the indexing model from the issue text.