[Android] Add documentation of Crosswalk for Android AAR

This documentation introduce about using crosswalk aar in Gradle and maven project. Add samples about using AAR with Gladle and Maven Project.

BUG=XWALK-2545
I think this is a little confusing.  You are saying to install gradle (part of the Android SDK?) but you are not saying how.  If you could give steps such as:   Find the gradlew executable in the Android SDK tools under gradle/wrapper/gradlew.   Navigate to the root of your project.   Run the following:    $ <path to SDK/tools>/gradle/wrapper/gradlew    (Is that what you are supposed to do?)