* Fix warning about deprecated `keepRules.file` API
The proguard files have been moved to `app/proguard` in several directories for `common`, `foss`, and `enterprise` rules and it's those directories that are added through `keepRules.addStaticSourceDirectory(dir)`.
* Use `findPropery` instead of the deprecated `project.properties["key"]` API for `allWarningsAsErrors`
* Remove very legacy .gitignore rule and restore proguard `.keep` rules
Jorge Martin Espinosa