Bump android-maven-gradle-plugin from 1.5 to 2.1

Bumps [android-maven-gradle-plugin](https://github.com/dcendents/android-maven-gradle-plugin) from 1.5 to 2.1.
- [Release notes](https://github.com/dcendents/android-maven-gradle-plugin/releases)
- [Commits](https://github.com/dcendents/android-maven-gradle-plugin/compare/1.5...2.1

)

Signed-off-by: dependabot[bot] <support@dependabot.com>