Update ProGuard rules to stay within dex method limit
This branch updates the ProGuard rules to stay within the dex method limit and adds an APK build to the CI pipeline so we can spot any ProGuard issues earlier in future.
The branch should be thoroughly tested, as org.h2 or org.briarproject classes that were previously kept may now be removed. I'm not too worried about the org.briarproject classes because the default Android rules should ensure that all components such as activities, fragments and broadcast receivers are kept, along with all classes that are transitively reachable from them. But H2 uses reflection internally, so we need to make sure we're keeping all the classes that H2 needs to load via reflection for our usage of H2.
Edited by akwizgran