Drone refactor project structure update and Android-studio compatibility

- turned Core project into an android library project. This is only a stub android library, and it can still be loaded as a regular java project. This was done to resolve dependency issues with gradle build.

- added gradle build file for the Core project.

- updated package name for the core project from 'org.droidplanner' to org.droidplanner.core'

- updated package name for the android project from 'org.droidplanner' to 'org.droidplanner.android'. The package name in the manifest file stays 'org.droidplanner' otherwise the app would show as a different app on the play store, and user device.

- refactored the project files to match the new package names.

- updated the whole project so it now build and run in android-studio.