Update mobile-services-android-call-custom-api.md

ApiOperationCallback needs to be included or the steps in this guide will fail on ADT 20131030 with Java 1.6 and API 18
Mobile Services: Adding call-custom-api-html and a reference to it in get-started-html
Changing reference to pip2.7 command
trying to fix merge conflict #1742
trying to fix merge conflict #1742
Merge pull request #1818 from tysonn/may12-14

May12 14