Let "adb connect" connect to emulators too

- adb can now connect to an emulator configured with an arbitrary
  pair of <console port, adb port>. These two ports do not have to be
  adjacent.
  This can be done from the commandline at any time using
  adb connect emu:<console_port>,<adb_port>
- Emulators running on ports outside the normal range
  (5554/5555-5584/5585) register themselves on startup if they follow
  the convention "console port+1==abd port".
- Emulators outside the normal port range will not be auto-detected on
  adb startup as these ports are not probed.
- The index into local_transports[] array in transport_local.c does no
  longer indicate the port number of the local transport. Use the altered
  atransport struct to get the port number.
- I have chosen not to document the adb connect emu:console_port,adb_port
  syntax on adb's help screen as this might be confusing to most readers
  and useful to very few.
- I don't expect this to introduce any (backwards) compatibility issues.

Change-Id: Iad3eccb2dcdde174b24ef0644d705ecfbff6e59d