Configure Dackka to generate javadoc for protected methods

#minor-release

PiperOrigin-RevId: 487497923
(cherry picked from commit a402955)

javadoc_combined.gradle

@@ -115,11 +115,16 @@ class CombinedJavadocPlugin implements Plugin<Project> {
             def sourcesString = project.files(sources.flatten())
                 .filter({ f -> project.file(f).exists() }).join(";")
             def dependenciesString = project.files(dependencies).asPath.replace(':', ';')
+            def sourceSet = [
+                "-src", sourcesString,
+                "-classpath", dependenciesString,
+                "-documentedVisibilities", "PUBLIC;PROTECTED"
+              ].join(" ")
             args("-moduleName", "",
                 "-outputDir", "$dackkaOutputDir",
                 "-globalLinks", "$globalLinksString",
                 "-loggingLevel", "WARN",
-                "-sourceSet", "-src $sourcesString -classpath $dependenciesString",
+                "-sourceSet", "$sourceSet",
                 "-offlineMode")
             environment("DEVSITE_TENANT", "androidx/media3")
           }