adb: Distinguish betwen already-connected and connection failure

This change returns a different value (-EALREADY) when a connection has
already been established, as opposed to a real connection failure (which
still returns -1).

Bug: 74411879
Test: Opened a socket, tried to adb connect to it,
      got "failed to connect to localhost:1337"

Change-Id: Ic216ddef7f28eb43ca750f9e51d068c077d54b07