Fix the demo build.gradle, so it now correctly skips signingconfig where necessary

[InnoLuke]

mobile/build.gradle provides a signingConfig for release builds. The data for the signingConfig was loaded from local.properties, if it existed, otherwise the signingConfig was skipped. The problem is that Android Studio auto-creates a local.properties, so the file always exists when gradle runs - even if you manually delete it.

The local.properties exists check is therefore useless, and results in failed builds when checking out the project locally, or running on Jitpack: Cause: path may not be null or empty string. path=''

This PR removes the useless exists check and replaces it with a check that all the necessary properties are present (non-null) before passing them to signingConfig.

I also cleaned up some duplicated plugin declarations.