Allow adb server port to be overriden by env var. Fixes calabash/calabash-android#265
Update the env var to `ANDROID_ADB_SERVER_PORT` to be more explicit about what it is.

This also matches up with what the Android Emulator Plugin sets
during a build execution on Jenkins.  See
https://wiki.jenkins-ci.org/display/JENKINS/Android+Emulator+Plugin#AndroidEmulatorPlugin-Environment
Merge pull request #2 from MeetMe/override-adb-port

Allow adb server port to be overriden by env var.