fixed an NPE in AndroidManifest.findLibraries() when "project.properties" does not exist in the baseDir

- updated the getProperties(FsFile propertiesFile) method so that it returns an empty Properties() object when the specified propertiesFile does not exist.
- updated the findLibraries() method to remove the null checks for local variables 'properties' and 'overrideProperties' since they will never be null because getProperties(FsFile propertiesFile) will now return an empty Properties object when the parameter propertiesFile does not exist.
- these changes allow for only providing a "test-project.properties" file without the need for an empty "project.properties" file in the baseDir.