Fix AppFuseMountScope.close()

The boolean tracking whether the scope had been opened or not
was never set before. Set it in open(), as was the case in the
previous (non-binder) version of this class.

Bug: 132901939
Test: monitor `adb shell mount` while browsing photos and
      verify the appfuse mounts don't build up over time
Change-Id: Ia164ac4209126ed7e4d273c5fca4ababc0c66f2a