Immediately destroy BluetoothSocket's on close().

Unfortunatley, shutdown() on the underlying fd does not actually stop a
listening socket from listening. You need to call close() on the fd to
do this. There is no way around it.

So this means the Java BluetoothSocket code has to call destroyNative() during
BluetoothSocket.close().

Since native methods cannot be called after destroyNative(), add a ReadWrite
lock and mClosed field to protect access to native methods.

This fixes the "resource busy" error when Bluetooth OPP and Bluetooth PBAP
tried to resume listening after turning BT off and then on.