DAVdroid have updated to kotlin, and kotlin is problematic with
annotation processors which was breaking the whole build.
This also meant we were not able to update to a new gradle version or
import just partial, smaller fixes. Now we have control over the
submodules as well, which fixes the above issues. At least for now.
This commit includes the major changes between DAVdroid and EteSync. It
adjusts the app to use the EteSync protocol and server. It includes some
ugliness still, and it's a squash of many ugly snapshot commits while
hacking on the initial DAVdroid code.
History should be "clean" from this point onwards.
* use cert4android instead of MemorizingTrustManager
* new app setting: distrust system certificates
* add network security config to manifest so that user-installed CAs will be accepted in Android 7 again
* update gradle
