Add allow_other mount option for appfuse.

After DocumentsProvider opens FD on app fuse, DocumentProvider passes it
to other applications. To allow other applications to use the FD on app
fuse, we need to specify allow_other mount option.

BUG=25756419

Change-Id: I3c729f90e5b822a7b1032bf80726cc234c0936b1

CommandListener.cpp

@@ -688,6 +688,7 @@ int CommandListener::AppFuseCmd::runCommand(SocketClient *cli,
                     "fd=%i,"
                     "rootmode=40000,"
                     "default_permissions,"
+                    "allow_other,"
                     "user_id=%d,group_id=%d",
                     device_fd,
                     uid,