Fix BluetoothSocket file descriptor leak

Sockets accepted on a server socket didn't populate
the mPfd field, which is used to close out the java
end of the native-and-java communication socket when
the overall rfcomm socket is closed. #badnewsbears

b/21398841

Change-Id: I3adb0a9965f83d0f3006fa4f79ea4abeab5c9a17