implementing support for fdroid app repos as app install source

[eighthave]

F-Droid (https://f-droid.org) provides a standard format for repositories of apps. There are many client apps that read repositories and install apps from them. hmdm-server could be a client. That would make it really easy to manage app installation in hmdm-server. There are a number of tools for managing fdroid-compatible repos (CLI fdroidserver Web GUI Repomaker etc). If hmdm-server can pull apps from fdroid repos, then it can automatically provide APKs and even pull updates.

There is a library for making it easy to download and verify the app index format:

        <dependency>
            <groupId>org.fdroid</groupId>
            <artifactId>download</artifactId>
            <version>0.1.1</version>
        </dependency>

I and other F-Droid contributors would be happy to help get this implemented. I've done plenty of Java for Android, but never Tomcat.