Android: Institute groovy style and lint guidance

Add the groovy style guide and linter resources (for VS code) to
//third_party/android_deps/README.md since this is the only project we
have that contains groovy code.

Re-format and fix our existing code to be more idiomatic and to conform
to the groovy code linter with the settings listed in the README.md.
This allows us to automatically fix-up style issues locally as we work
on the code, and also makes it easier for new code to be more idomatic
when extending our existing code.

Bug: 1216032
Cq-Include-Trybots: luci.chromium.try:3pp-linux-amd64-packager
Change-Id: Ie9a848912d69233df462f749406765622ddeb3ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2940907
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#889727}

third_party/android_deps/README.md

@@ -71,4 +71,44 @@ features. An alternative way to implement it is to mix gradle to purely fetch
 dependencies and their pom.xml files, and use Python to process and generate
 the files. This approach was not as successful, as some information about the
 dependencies does not seem to be available purely from the POM file, which
-resulted in expecting dependencies that gradle considered unnecessary.
+resulted in expecting dependencies that gradle considered unnecessary. This is
+especially true nowadays that pom.xml files for many dependencies are no longer
+maintained by the package authors.
+
+#### Groovy Style Guide
+The groovy code in `//third_party/android_deps/buildSrc/src/main/groovy` loosely
+follows the [Groovy Style Guide][groovy_style_guide], and can be linted by each
+dev with [npm-groovy-lint][npm_groovy_lint] via the
+[VS Code extension][vs_code_groovy_lint]. The line length limit for groovy is
+**120** characters.
+
+Here is a sample `.groovylintrc.json` file:
+
+```
+{
+    "extends": "recommended",
+    "rules": {
+        "CatchException": "off",
+        "CompileStatic": "off",
+        "DuplicateMapLiteral": "off",
+        "DuplicateNumberLiteral": "off",
+        "DuplicateStringLiteral": "off",
+        "FactoryMethodName": "off",
+        "JUnitPublicProperty": "off",
+        "JavaIoPackageAccess": "off",
+        "MethodCount": "off",
+        "NestedForLoop": "off",
+        "ThrowRuntimeException": "off",
+        "formatting.Indentation": {
+            "spacesPerIndentLevel": 4
+        }
+    }
+}
+```
+
+This is a list of rule names: [Groovy Rule Index by Name][groovy_rule_index].
+
+[groovy_style_guide]: https://groovy-lang.org/style-guide.html
+[npm_groovy_lint]: https://github.com/nvuillam/npm-groovy-lint
+[vs_code_groovy_lint]: https://marketplace.visualstudio.com/items?itemName=NicolasVuillamy.vscode-groovy-lint
+[groovy_rule_index]: https://codenarc.org/codenarc-rule-index-by-name.html