Add buildDir to java patch-module paths.

This solves a problem where javac fails to find patch-module targets
when a build diretory is not under the project root.

Bug: 117770838
Test: java_test.go + manual build
Change-Id: If70d0d388a3f6c87b3f78a927df2063616d50c8a

java/java.go

@@ -1011,8 +1011,15 @@ func (j *Module) collectBuilderFlags(ctx android.ModuleContext, deps deps) javaB
 	}
 
 	if j.properties.Patch_module != nil && flags.javaVersion == "1.9" {
-		patchClasspath := ".:" + flags.classpath.FormJavaClassPath("")
-		javacFlags = append(javacFlags, "--patch-module="+String(j.properties.Patch_module)+"="+patchClasspath)
+		// Manually specify build directory in case it is not under the repo root.
+		// (javac doesn't seem to expand into symbolc links when searching for patch-module targets, so
+		// just adding a symlink under the root doesn't help.)
+		patchPaths := ".:" + ctx.Config().BuildDir()
+		classPath := flags.classpath.FormJavaClassPath("")
+		if classPath != "" {
+			patchPaths += ":" + classPath
+		}
+		javacFlags = append(javacFlags, "--patch-module="+String(j.properties.Patch_module)+"="+patchPaths)
 	}
 
 	// systemModules