[java-source-utils] Fix lgtm java/path-injection-local (dotnet#1079)

Context: e11d024

Commit e11d024 attempted to fix LGTM-reported
[`java/path-injection-local`][0] warnings by using the comment
`// lgtm [java/path-injection-local]`.

Unfortunately, this is insufficient: the comment *also* needs to
provide a 25+ character justification for why the offending statement
can be ignored.  This justification was not provided.

Update the `// lgtm [java/path-injection-local]` comments to provide
a justification, as required by tooling.

Copying the longer justification from e11d024:

> LGTM is complaining that `tools/java-source-utils` (69e1b80) accepts
> user-controlled data.  These warnings will be *ignored* because the
> app is *unusable* without "user-controlled data"
> …
> These are all user-controlled, and they are necessary to allow
> `java-source-utils` to *work*.
> …
> LGTM complains that `--output-javadoc FILE` accepts a user-controlled
> path which may [contain] directory separator chars, and
> *this is intentional*; using it would be annoying if that weren't true!

See also [`JavaSourceUtils.cs`][1], which passes [a value][2] located
within `$(IntermediateOutputPath)` to
`java-source-utils.jar --output-javadoc`.  Allowing `--output-javadoc`
to contain directory separator chars is what makes this possible!

[0]: https://github.com/github/codeql/blob/f192191e8c4c14d70a86342de47c8882516c7c25/java/ql/src/Security/CWE/CWE-022/TaintedPath.qhelp
[1]: https://github.com/xamarin/xamarin-android/blob/b00185c485287c2c5f0350a067ebc178aec2382c/src/Xamarin.Android.Build.Tasks/Tasks/JavaSourceUtils.cs#L134-L135
[2]: https://github.com/xamarin/xamarin-android/blob/b00185c485287c2c5f0350a067ebc178aec2382c/src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Bindings.ClassParse.targets#L69

tools/java-source-utils/src/main/java/com/microsoft/android/JavaSourceUtilsOptions.java

@@ -167,7 +167,7 @@ private final JavaSourceUtilsOptions parse(Iterator<String> args) throws IOExcep
 					final   String          bootClassPath   = getNextOptionValue(args, arg);
 					final   ArrayList<File> files           = new ArrayList<File>();
 					for (final String cp : bootClassPath.split(File.pathSeparator)) {
-						final   File    file    = new File(cp); // lgtm [java/path-injection-local]
+						final   File    file    = new File(cp); // lgtm [java/path-injection-local] java-source-utils.jar is a command-line app, and is useless if it doesn't support command-line args.
 						if (!file.exists()) {
 							System.err.println(App.APP_NAME + ": warning: invalid file path for option `-bootclasspath`: " + cp);
 							continue;
@@ -253,7 +253,7 @@ private final JavaSourceUtilsOptions parse(Iterator<String> args) throws IOExcep
 					if (arg.startsWith("@")) {
 						// response file?
 						final   String  responseFileName = arg.substring(1);
-						final   File    responseFile     = new File(responseFileName);  // lgtm [java/path-injection-local]
+						final   File    responseFile     = new File(responseFileName);  // lgtm [java/path-injection-local] java-source-utils.jar is a command-line app, and is useless if it doesn't support command-line args.
 						if (responseFile.exists()) {
 							final   Iterator<String>        lines   =
 								Files.readAllLines(responseFile.toPath())
@@ -267,7 +267,7 @@ private final JavaSourceUtilsOptions parse(Iterator<String> args) throws IOExcep
 							break;
 						}
 					}
-					final   File    file        = new File(arg);    // lgtm [java/path-injection-local]
+					final   File    file        = new File(arg);    // lgtm [java/path-injection-local] java-source-utils.jar is a command-line app, and is useless if it doesn't support command-line args.
 					if (!file.exists()) {
 						System.err.println(App.APP_NAME + ": warning: invalid file path for option `FILES`: " + arg);
 						break;
@@ -347,7 +347,7 @@ static File getNextOptionFile(final Iterator<String> args, final String option)
 			throw new IllegalArgumentException(
 					"Expected required value for option `" + option + "`.");
 		final   String  fileName    = args.next();
-		final   File    file        = new File(fileName);   // lgtm [java/path-injection-local]
+		final   File    file        = new File(fileName);   // lgtm [java/path-injection-local] java-source-utils.jar is a command-line app, and is useless if it doesn't support command-line args.
 		if (!file.exists()) {
 			System.err.println(App.APP_NAME + ": warning: invalid file path for option `" + option + "`: " + fileName);
 			return null;

tools/java-source-utils/src/main/java/com/microsoft/android/JavadocXmlGenerator.java

@@ -39,7 +39,7 @@ public JavadocXmlGenerator(final String output) throws FileNotFoundException, Pa
 		if (output == null)
 			this.output = System.out;
 		else {
-			final File file     = new File(output);     // lgtm [java/path-injection-local]
+			final File file     = new File(output);     // lgtm [java/path-injection-local] java-source-utils.jar is a command-line app, and is useless if it doesn't support command-line args.
 			final File parent   = file.getParentFile();
 			if (parent != null) {
 				parent.mkdirs();