Update document:retrofit use gradle as build tool,maven cmd not works.

Added RxJava3 adapter

OkHttp 3.14.9

This contains the fix for Android detection on the OkHttp side.

Move source/target config out of error-prone block

Error-prone only works on pre-12 at the moment and we need this configuration to apply for all JDKs.

Use system properties to detect Android

Android classes can show up on the classpath on the JvM in places like IntelliJ when you have the Android plugin applied.

Add Android integration tests

Switch to GitHub Actions

Add google-java-format and reformat

Convert project to Gradle

This allows integrating an Android test project easily.

Small fixes from error-prone and animal sniffer

Small fixes from error-prone

Don't attempt to resolve Lookup ctor on API 24/25