Make javaVersion an enum

Remove the hardcoded checks against "1.9" by making javaVersion
an enum and implementing javaVersion.usesJavaModules().

Test: TestClasspath
Change-Id: I559eeb1f45880bb8177269c6d977ee4dfbadce57
diff --git a/java/droiddoc.go b/java/droiddoc.go
index 1091331..32758f6 100644
--- a/java/droiddoc.go
+++ b/java/droiddoc.go
@@ -665,7 +665,7 @@
 	cmd := javadocSystemModulesCmd(ctx, rule, j.srcFiles, outDir, srcJarDir, srcJarList,
 		deps.systemModules, deps.classpath, j.sourcepaths)
 
-	cmd.FlagWithArg("-source ", javaVersion).
+	cmd.FlagWithArg("-source ", javaVersion.String()).
 		Flag("-J-Xmx1024m").
 		Flag("-XDignore.symbol.file").
 		Flag("-Xdoclint:none")
@@ -1432,12 +1432,12 @@
 	}
 }
 
-func metalavaCmd(ctx android.ModuleContext, rule *android.RuleBuilder, javaVersion string, srcs android.Paths,
+func metalavaCmd(ctx android.ModuleContext, rule *android.RuleBuilder, javaVersion javaVersion, srcs android.Paths,
 	srcJarList android.Path, bootclasspath, classpath classpath, sourcepaths android.Paths) *android.RuleBuilderCommand {
 	cmd := rule.Command().BuiltTool(ctx, "metalava").
 		Flag(config.JavacVmFlags).
 		FlagWithArg("-encoding ", "UTF-8").
-		FlagWithArg("-source ", javaVersion).
+		FlagWithArg("-source ", javaVersion.String()).
 		FlagWithRspFileInputList("@", srcs).
 		FlagWithInput("@", srcJarList)