Add a linter

Add a linter to verify (and format) our code to a certain standard.

• Set up https://github.com/sherter/google-java-format-gradle-plugin linter. (Follow the Quick Start section.)
• Use AOSP style. (Check the Extended Usage section in the link above.)
• Verify all .java files in the project are formatted properly.
  • If we found any files not formatted properly, then format it using the linter.
• If we already have a readme file (see #2), then update it to include two commands:
  • how to verify all .java files are formatted properly;
  • how to format all .java files.