Media upload tweaks (#3643)

* Focus the media caption composer if a hardware keyboard is available
Not worrying about live connections, this screen is short lived.

* Fix a bug where you could hit the send button multiple times while waiting for the media to be converted.

* Begin processing media as soon as the media upload screen is shown.

ElementX/Sources/Screens/MediaUploadPreviewScreen/MediaUploadPreviewScreenViewModelProtocol.swift

@@ -11,4 +11,7 @@ import Combine
 protocol MediaUploadPreviewScreenViewModelProtocol {
     var actions: AnyPublisher<MediaUploadPreviewScreenViewModelAction, Never> { get }
     var context: MediaUploadPreviewScreenViewModelType.Context { get }
+    
+    /// Stops any ongoing media processing tasks.
+    func stopProcessing()
 }