[Java] Add jacoco Gradle plugin for code coverage reporting

Adds the jacoco plugin for code coverage reporting and enforcement.

Code coverage reports can be generated with the jacocoTestReport task:

./gradlew test jacocoTestReport

The resulting report can be found under build/reports/jacoco/.

Later, we can consider enabling enforcement of code coverage
standards using the jacocoTestCoverageVerification task.

For more details, see https://docs.gradle.org/current/userguide/jacoco_plugin.html.

Bug: T288742
Change-Id: I3eed50f358ef3765e2111d1817be6a6fdac2e07c