Make sure we clean up the pre-processed and uploaded media (#5039)

* Add `MediaSender.cleanUp()` and `MediaPreProcessor.cleanUp()` methods: this will remove the temporary files created when pre-processing media before sending them.

* Make sure we clean up also the previous temporary media.

* Fix the condition for the custom back handler in the attachments preview screen

* Tests: check the clean up is performed when needed

libraries/mediaupload/test/src/main/kotlin/io/element/android/libraries/mediaupload/test/FakeMediaPreProcessor.kt

@@ -26,6 +26,9 @@ class FakeMediaPreProcessor(
     var processCallCount = 0
         private set
 
+    var cleanUpCallCount = 0
+        private set
+
     private var result: Result<MediaUploadInfo> = Result.success(
         MediaUploadInfo.AnyFile(
             File("test"),
@@ -108,4 +111,8 @@ class FakeMediaPreProcessor(
             )
         )
     }
+
+    override fun cleanUp() {
+        cleanUpCallCount += 1
+    }
 }