api: Ignore ClassCastExceptions for hard-coded providers

This was observed in the Bazel/Blaze build where io.grpc.util is a
separate target from the rest of core. During the build of a library
SecretRoundRobinLoadBalancerProvider was not on the classpath, and the
library was later included into a binary using grpc-core from Maven
Central which includes SecretRoundRobinLoadBalancerProvider.

```
java.util.ServiceConfigurationError: Provider io.grpc.util.SecretRoundRobinLoadBalancerProvider$Provider could not be instantiated java.lang.ClassCastException: class io.grpc.util.SecretRoundRobinLoadBalancerProvider$Provider cannot be cast to some.app.aaa.aab
```

api/src/main/java/io/grpc/ServiceProviders.java

@@ -122,15 +122,26 @@ public static <T> Iterable<T> getCandidatesViaServiceLoader(Class<T> klass, Clas
   static <T> Iterable<T> getCandidatesViaHardCoded(Class<T> klass, Iterable<Class<?>> hardcoded) {
     List<T> list = new ArrayList<>();
     for (Class<?> candidate : hardcoded) {
-      list.add(create(klass, candidate));
+      T t = createForHardCoded(klass, candidate);
+      if (t == null) {
+        continue;
+      }
+      list.add(t);
     }
     return list;
   }
 
-  @VisibleForTesting
-  static <T> T create(Class<T> klass, Class<?> rawClass) {
+  private static <T> T createForHardCoded(Class<T> klass, Class<?> rawClass) {
     try {
       return rawClass.asSubclass(klass).getConstructor().newInstance();
+    } catch (ClassCastException ex) {
+      // Tools like Proguard that perform obfuscation rewrite strings only when the class they
+      // reference is known, as otherwise they wouldn't know its new name. This means some
+      // hard-coded Class.forNames() won't be rewritten. This can cause ClassCastException at
+      // runtime if the class ends up appearing on the classpath but that class is part of a
+      // separate copy of grpc. With tools like Maven Shade Plugin the class wouldn't be found at
+      // all and so would be skipped. We want to skip in this case as well.
+      return null;
     } catch (Throwable t) {
       throw new ServiceConfigurationError(
           String.format("Provider %s could not be instantiated %s", rawClass.getName(), t), t);

api/src/test/java/io/grpc/ServiceProvidersTest.java

@@ -215,19 +215,35 @@ public void getCandidatesViaHardCoded_failAtInit_moreCandidates() throws Excepti
   }
 
   @Test
-  public void create_throwsErrorOnMisconfiguration() throws Exception {
-    class PrivateClass {}
+  public void getCandidatesViaHardCoded_throwsErrorOnMisconfiguration() throws Exception {
+    class PrivateClass extends BaseProvider {
+      private PrivateClass() {
+        super(true, 5);
+      }
+    }
 
     try {
-      ServiceProviders.create(
-          ServiceProvidersTestAbstractProvider.class, PrivateClass.class);
+      ServiceProviders.getCandidatesViaHardCoded(
+          ServiceProvidersTestAbstractProvider.class,
+          Collections.<Class<?>>singletonList(PrivateClass.class));
       fail("Expected exception");
     } catch (ServiceConfigurationError expected) {
-      assertTrue("Expected ClassCastException cause: " + expected.getCause(),
-          expected.getCause() instanceof ClassCastException);
+      assertTrue("Expected NoSuchMethodException cause: " + expected.getCause(),
+          expected.getCause() instanceof NoSuchMethodException);
     }
   }
 
+  @Test
+  public void getCandidatesViaHardCoded_skipsWrongClassType() throws Exception {
+    class RandomClass {}
+
+    Iterable<ServiceProvidersTestAbstractProvider> candidates =
+        ServiceProviders.getCandidatesViaHardCoded(
+            ServiceProvidersTestAbstractProvider.class,
+            Collections.<Class<?>>singletonList(RandomClass.class));
+    assertFalse(candidates.iterator().hasNext());
+  }
+
   private static class BaseProvider extends ServiceProvidersTestAbstractProvider {
     private final boolean isAvailable;
     private final int priority;