AdbStream.java: isClosed() should be synchronous

[Krush206]

Issues arise if this isn't called synchronously.

{
  AdbStream stream = connection.open("shell:uname");

  while(!stream.isClosed())
    ;
}

results in an infinite loop, even though the variable is set true.

[Krush206]

Refer AdbConnection.java to https://github.com/cgutman/AdbLib/pull/18.