Verify Gradle Wrapper files in CI / Github Action

[emanuelb]

The repo contain pre-built binary gradle-wrapper.jar at:
android/gradle/wrapper/gradle-wrapper.jar
and related gradle-wrapper scripts at:
android/gradlew
android/gradlew.bat

As they inserted into the repo directly (as source) without package manager, a verification step is needed to ensure a malicious change wasn't introduced to them, see:

Verifying Gradle Wrappers with GitHub Actions
https://blog.gradle.org/gradle-wrapper-checksum-verification-github-action
https://docs.gradle.org/current/userguide/gradle_wrapper.html#wrapper_checksum_verification

To fix this issue, Gradle developed the "Gradle Wrapper Validation" Github action:
https://github.com/marketplace/actions/gradle-wrapper-validation

Which may add in the future verification for scripts as well, see open issue: "Validate gradlew scripts"
gradle/actions#283