android: Add android build on travis-ci

This adds a android-apk-build which runs on travis-ci. This is using a
quite ugly trick, building in a docker container, basically just to get
a newer cmake. The cmake in trusty is way to old to work with android
builds.

A good side-effect is that this is a complete copy-paste for anyone who
would like to build android-binaries them self on Linux. All the
uglyness is hidden away in a docker container.

Signed-off-by: Anton Lundin <glance@acc.umu.se>

scripts/android/before_install.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+# Travis only pulls shallow repos. But that messes with git describe.
+# Sorry Travis, fetching the whole thing and the tags as well...
+git fetch --unshallow
+git pull --tags
+git describe
+
+# Ugly, but keeps it running during the build
+docker run -v $PWD:/workspace/subsurface --name=builder -w /workspace -d ubuntu /bin/sleep 60m
+docker exec -t builder apt-get update
+# subsurface android build dependencies
+docker exec -t builder apt-get install -y git cmake autoconf libtool-bin openjdk-8-jdk-headless ant wget unzip python bzip2 pkg-config
+# Qt installer dependencies
+docker exec -t builder apt-get install -y libx11-xcb1 libgl1-mesa-glx libglib2.0-0
+# Inject cached 3pp's (if none exists in 3pp dir, we inject zero ones, and all is downloaded in the container)
+# TODO: caching
+#docker cp 3pp builder:/workspace
+docker exec -t builder mkdir -p /workspace/3pp