Fix AppFuse deadlock.

        @Override

        public void close() throws Exception {

            if (mMounted) {

                BackgroundThread.getHandler().post(() -> {

                    try {

                        // We need to run the unmount on a separate thread to

                        // prevent a possible deadlock, where:

                        // 1. AppFuseThread (this thread) tries to call into vold

                        // 2. the vold lock is held by another thread, which called:

                        //    mVold.openAppFuseFile()

                        //    as part of that call, vold calls open() on the

                        //    underlying file, which is a call that needs to be

                        //    handled by the AppFuseThread, which is stuck waiting

                        //    for the vold lock (see 1.)

                        // It is safe to do the unmount asynchronously, because the mount

                        // path we use is never reused during the current boot cycle;

                        // see mNextAppFuseName. Also,we have anyway stopped serving

                        // requests at this point.

                        mVold.unmountAppFuse(uid, mountId);

                    } catch (RemoteException e) {

                        throw e.rethrowAsRuntimeException();

                    }

                });

                mMounted = false;

            }

        }