Fix platform_compat-test-rules under Java language level 9.

This java_library_static (introduced in https://r.android.com/1113075)
builds classes which are in the Java module java.base. To avoid a
split packages error when building with Java language level 9, it
needs a patch_module attribute. (This seems appropriate since it these
are rules for testing code that is in java.base.)

Test: EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9=true make platform_compat-test-rules
Bug: 115604102
Change-Id: I966751cc6d90ab97a67b2144ab0c787ab11c4f09
diff --git a/JavaLibrary.bp b/JavaLibrary.bp
index afba8e9..b61ed44 100644
--- a/JavaLibrary.bp
+++ b/JavaLibrary.bp
@@ -503,6 +503,8 @@
         "android-support-test",
     ],
     platform_apis: true,
+    // This builds classes that are in the java.base Java module:
+    patch_module: "java.base",
 }
 
 // Builds the core-tests-support library used by various tests.