Add support for native shared libraries.

The builder searches the target/native directory for .so files. If it
finds any:

* Copy them to the target location.
* Add a java source file to load the libraries.
* Modify the manifest to use the new java activity.

This is required as the normal android shared library loader
only looks by default in /system/lib and /vendor/lib for shared
libraries, meaning that you have to manually load them via
System.loadLibrary().