Convert server to an Android project

To simplify the device server-side build, use gradle to create an APK,
even if we use it as a simple jar, by running its main() method.

server/.gitignore

@@ -1,4 +1,8 @@
-*.class
-*.jar
-*.dex
-/gen
+*.iml
+.gradle
+/local.properties
+/.idea/
+.DS_Store
+/build
+/captures
+.externalNativeBuild