CMFileManager: account for displayed dialogs during activity tear down

Repro:
 - Start copying a large file (>100 MB)
 - Leave CMFM and trigger a config change
   (e.g. set text size to small in Settings -> Display)
 - Return to CMFM
 - Observe: window leak in logcat

Change-Id: Ic875d4f86edf0446b889e6442126bd76a692a7c6