travis: improve docker images generation for building

Make a build heriarchy to reduce images size.
Add Android Thing Build.

Signed-off-by: Nicolas Oliver <dario.n.oliver@intel.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

docker/Dockerfile.java

@@ -0,0 +1,14 @@
+FROM mraa-base
+
+# Update apt-get
+RUN apt-get -y update && \
+  # Java Build Dependencies
+  apt-get -y --no-install-recommends install default-jre default-jdk
+
+# Set JAVA_HOME
+ENV JAVA_HOME /usr/lib/jvm/java-8-openjdk-amd64/
+
+# Set Workdir
+WORKDIR $MRAA_SRC_DIR
+
+CMD bash