Now ignores IntelliJ's .idea directories
Fixes wrong path for dx and aapt tools

Because Google relocated some of the platform-tools into the
build-tools for easier incremental updates in the future this broke
where those tools are located.

Updated the android-maven-plugin to version 3.6.0 which was the first
release where the new structure was supported.

Also update the path itself to the dx tool to point to
build-tools/20.0.0/lib/dx.jar.  So this mean that you'll need build-tools
20.0.0 when building this solution.