Prevent loop when exiting the attachments preview screen (#5078)

Disable the custom `BackHandler` when the `sendState` is `Done`. Otherwise, it can get into a loop in some cases where `postCancel()` calls the `OnDoneListener`, which triggers an up navigation, which then simulates a back press... and we end up in the custom `BackHandler` again.

Author: jmartinesp

features/messages/impl/src/main/kotlin/io/element/android/features/messages/impl/attachments/preview/AttachmentsPreviewView.kt

@@ -66,7 +66,7 @@ fun AttachmentsPreviewView(
         state.eventSink(AttachmentsPreviewEvents.CancelAndClearSendState)
     }
 
-    BackHandler(enabled = state.sendActionState !is SendActionState.Sending.Uploading) {
+    BackHandler(enabled = state.sendActionState !is SendActionState.Sending.Uploading && state.sendActionState !is SendActionState.Done) {
         postCancel()
     }