ics-openvpn is now officially on GitHub, and they track openssl and
openvpn as submodules, so it's easier to update everything. Just a git
submodule update --recursive.

I've also set up soft links to native modules from ics-openvpn in app,
so that we don't copy files in Gradle (which was causing problems with
the submodules .git* files, not being copied). That makes the repo
cleaner.

This return to "app" instead of "bitmask_android" is due to this reading: https://developer.android.com/sdk/installing/studio-build.html#projectStructure
I'll have to tweak the final apk name in build.gradle.

This way, gradle commands generate apks correctly named.