Add service for signing Android app bundles

While documenting the workflow for publishing a new app on Google Play I realized that one needs to upload at least one AAB manually. This AAB needs to be signed with our upload key. To allow people without access to our upload key to add new apps to Google Play I added this service by factoring the AAB signing out of the googleplaypublisher. googleplaypublisher still signs the AABs itself so that the new service is only needed for the manual upload of AABs.