Jenkins build android

Adds

* build.xml file that can be used for building the JNI layer
* adds a pre-compile step to build.xml to build the C++ code first
* treats the JNI layer as an Android library project

I think, but did not try, that the pre-compile step could be moved around based on how developers want to use it, if they want to use it, but for now it seems like the JNI headers are being checked into the repository (:+1:) but building the C++ code first would not work when there are new methods added to the JNI.

So I am proposing this unless some of the Android developers would like it to be changed, and I would be happy to make the change to suit the developers needs! :smile: 

(There is a jenkins plugin that will generate this script but I was not sure if/how it could be modified after it was generated so I opted to just generate it now and get it checked into the repo to avoid that problem)

@rkwright, are you still keeping the develop branch from changing? If so this can wait to be merged until you are ready!