android.add_jars config option

This adds an android.add_jars config option to buildozer.spec

it is a semicolon-separated list of jar filenames (with wildcard support) that will be included inside the apk bundle's libs directory, making their classes available to pyjnius.

This is a wrapper around the --add-jar command line option for python-for-android's build.py script from pull request https://github.com/kivy/python-for-android/pull/120 so naturally, there is no point in merging this one until after that PR has been considered :)