Fix Proxy.java and :jre_tests_ios

PiperOrigin-RevId: 538406338

Author: j2objc-copybara

jre_emul/android/platform/libcore/ojluni/src/main/java/java/lang/reflect/Proxy.java

@@ -999,9 +999,9 @@ private static void checkNewProxyPermission(Class<?> caller, Class<?> proxyClass
      *          {@code false} otherwise
      * @throws  NullPointerException if {@code cl} is {@code null}
      */
-    public static boolean isProxyClass(Class<?> cl) {
-        return Proxy.class.isAssignableFrom(cl) && proxyClassCache.containsValue(cl);
-    }
+    public static native boolean isProxyClass(Class<?> cl) /*-[
+      return [nil_chk(cl) isKindOfClass:[IOSProxyClass class]];
+    ]-*/;
 
     /**
      * Returns the invocation handler for the specified proxy instance.