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/MediaUploadPreviewScreenCoordinator.swift

@@ -52,6 +52,10 @@ final class MediaUploadPreviewScreenCoordinator: CoordinatorProtocol {
             .store(in: &cancellables)
     }
     
+    func stop() {
+        viewModel.stopProcessing()
+    }
+    
     func toPresentable() -> AnyView {
         AnyView(MediaUploadPreviewScreen(context: viewModel.context))
     }