Improve the gradle build process for custom Kiwix apps

[julianharty]

I've been learning how to build custom Android apps using gradle as part of trying to address a couple of small bugs. Based on these experiences and some discussions with various team members at the hackathon in Stockholm I'd like to help improve the gradle scripts that underpin other build scripts and tools.

I hope I/we don't introduce problems for those tools and would appreciate help testing any changes.

Some related issues

• Fixes for build-custom-app #1218
• gen-custom-android-build.py should provide a solution to override info.json #35
• Implement a automod system for the Android custom app version_name #32
• How can I build an android custom app for offline deployment kiwix-android-custom#66

[julianharty]

See also #900 for an example of building a custom app using another approach. Similarly kiwix/kiwix-android-custom#66 contains detailed guidance of building custom apps as of 2017