Create reports per variant

When running multiple `connectedAndroidCoverageTest[variant_name]` the plugin will create separate folders for each variant. Moreover, `ReportTask` was extracted. The plugin will not generate "false" coverage, as the `ReportTask` and `InstrumentationTask` excludes untestable classes. 

This PR fixes #1 , fixes #4 , fixes #5 , fixes #6 , fixes #7.