Fix for issue 5712743 video editor functional test : testPreviewDuringExport fails

-Correct the invalid filename, which contains control character.
 And with the invalid filename, the cts fails only on crespo, but passes on stingray and prime.

Change-Id: I77862e55e8ee990f72f617a804f26fd724914b5b

Author: Hong Teng

media/tests/MediaFrameworkTest/src/com/android/mediaframeworktest/functional/videoeditor/VideoEditorPreviewTest.java

@@ -709,7 +709,7 @@ public void onProgress(Object item, int action, int progress) {
 
 
         blockTillPreviewCompletes.acquire();
-                    final String fileName = mVideoEditor.getPath() + "\test.3gp";
+                    final String fileName = mVideoEditor.getPath() + "/test.3gp";
                     final int height = MediaProperties.HEIGHT_480;
                     final int bitrate = MediaProperties.BITRATE_512K;