put the jacoco.gradle file below somewhere in your app repo (like scripts folder), and then add apply from: '../scripts/jacoco.gradle' to your module gradle file.
This will add additional gradle build tasks to your app, in the form of "testFlavourUnitTestCoverage" that you can run manually (e.g. testStagingDebugUnitTestCoverage) to generate the coverage.
You can edit def excludes definition to further exclude classes you don't want to generate the coverage for. If you have source in other folders beyond those specified in def coverageSourceDirs, add them.
This will generate both exec file used by external code quality analyzers (like Sonarqube), in the build/jacoco folder, and XML and HTML reports, in the build/reports/jacoco/flavourName folder.