Add a core-util-jvm module.

This is basically a location where we can put common utils that can also
be imported by libsignal-service (which is java-only, no android
dependency).

app/src/main/java/org/thoughtcrime/securesms/webrtc/audio/IncomingRinger.java

@@ -8,7 +8,6 @@ import android.media.MediaPlayer;
 import android.net.Uri;
 import android.os.Build;
 import android.os.Vibrator;
-import android.provider.Settings;
 
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;