Pass config files to metalava

Adds a filegroup "metalava-config-files" whose contents are passed to
Metalava using the `--config-file` option.

Bug: 354699349
Test: m checkapi
Change-Id: I1e246517c2ef678e41214c975aecee97f9faca67
diff --git a/java/testing.go b/java/testing.go
index 5ae326d..7a42e4c 100644
--- a/java/testing.go
+++ b/java/testing.go
@@ -52,6 +52,8 @@
 	android.MockFS{
 		// Needed for linter used by java_library.
 		"build/soong/java/lint_defaults.txt": nil,
+		// Needed for java components that invoke Metalava.
+		"build/soong/java/metalava/Android.bp": []byte(`filegroup {name: "metalava-config-files"}`),
 		// Needed for apps that do not provide their own.
 		"build/make/target/product/security": nil,
 		// Required to generate Java used-by API coverage