Features/629+826 unittest mozbuild updates

This fixes #629 to add JUnit, robolectric and jococo coverage analysis
#826 needed a unittest around the Updater code, but that's actually going away because of the play store, so this actually closes off #862 as well.

I need to add some documentation for this, but the short of it is that the Makefile has reasonable targets.

The default `all` target will build a release.
`test` will compile and run unittests

There is now a split between debug and unittest builds as robolectric puts extra constraints on what is possible.