adb: Check sender's socket id when receiving packets.

handle_packet() in adb.c didn't check that when an A_WRTE packet is
received, the sender's local-id matches the socket's peer id.

This meant that a compromised adbd server could sent packets to
the host adb server, spoofing the identity of another connected
device if it could "guess" the right host socket id.

This patch gets rid of the issue by enforcing even more checks
to ensure that all packets comply with the description in
protocol.txt.

+ Fix a bug where closing a local socket associated with a
  remote one would always send an A_CLSE(0, remote-id, "")
  message, though protocol.txt says that should only happen
  for failed opens.

  The issue was that local_socket_close() called
  remote_socket_close() after clearing the remote socket's
  'peer' field.

  The fix introduces a new asocket optional callback,
  named 'shutdown' that is called before that, and is
  used to send the A_CLSE() message with the right ID
  in remote_socket_shutdown().

  Also add some code in handle_packet() to detect
  invalid close commands.

Change-Id: I9098bc8c6e81f8809334b060e5dca4fc92e6fbc9