Add getAdbServerPort function to adb

Needed for #3717 fix
'ANDROID_ADB_SERVER_PORT' is an optional environmental variable.
If you set it to a port number and start adb it will use this port.
If it's not set then adb will use port 5037.

http://stackoverflow.com/questions/3836571/changing-default-port-i-e-5037-on-which-adb-server-runs