Added new Config value for libraries

This is a potential solution to #1196. It removes the requirement to maintain a project.properties file (though it will still fall back to reading one, so existing functionality is preserved).

This config value specifies the paths to required Android Library projects (relative to the project base dir). When present, it supercedes the libraries from project.properties.

Also added a few constants in AndroidManifest.java to replace hard-coded strings.