dx: Minor code clean-up

A few automated code clean-ups:
- Insert missing @Override annotations.
- Make fields final where possible.
- Use StringBuilder rather StringBuffer for local use.
- Clean-up imports.

Bug: 65231722
Test: dx/tests/run-all-tests
Change-Id: I49810f1d43cb53ad32636a66eda2fadc538ba249
187 files changed