Update MediaPickerActivity.cs
Update MediaPickerActivity.cs

Following Scastria comment from the bug -
https://github.com/jamesmontemagno/MediaPlugin/issues/598#issuecomment-412937648
If we null out the completionsource on sleep, which will call OnDestry to the media Activity, it fixes the issue
jamesmontemagno/MediaPlugin,jamesmontemagno/MediaPlugin