Add Gradle build configuration file.

This is an initial support for Gradle. Most of all, this configuration
is about giving developers opportunity to contribute to Android app
using Android Studio IDE. At moment all dependencies are local and
distributed via the SDK.

Change-Id: I323f81cf1fad3511eefea810d2360b20205181de
Reviewed-on: https://gerrit.libreoffice.org/8017
Tested-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>