Updated Android dependencies

Updates a few Android dependencies (Jetty, Maven Plugin) and switches to the .aar version of the seamless package.
More info why apklib is deprecated: https://code.google.com/p/maven-android-plugin/wiki/ApkLib

Should only be merged, once https://github.com/4thline/seamless/pull/6 is merged & re-released to maven (switched to .aar format).