Updated the Gradle Build System.

I refreshed the gradle build system to the newest android standards (and
adjusted the README accordingly). I moved all the android application
code to a separate "mobile" module, which should make it easier to
handle later, when somone eventually adds support for Android Wear
devices (it's code will go into a "wear" module). This structure is
basically the structure that Android Studio makes after creating a new
project.

Change-Id: I7c7913dc9bfa3ac99ff8e738495c0ae811c6738f
Reviewed-on: https://gerrit.libreoffice.org/10147
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Tested-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>