Expose ServiceManager.waitForService as system API.

ART module needs that method in order to connect to artd.

Bug: 233915142
Test: m
Change-Id: I3fc487476da8d88238b6974707747b429a132909

core/api/module-lib-current.txt

@@ -358,6 +358,7 @@ package android.os {
   public final class ServiceManager {
     method public static boolean isDeclared(@NonNull String);
     method @Nullable public static android.os.IBinder waitForDeclaredService(@NonNull String);
+    method @Nullable public static android.os.IBinder waitForService(@NonNull String);
   }
 
   public class StatsServiceManager {

core/java/android/os/ServiceManager.java

@@ -276,6 +276,8 @@ public final class ServiceManager {
      * @return {@code null} only if there are permission problems or fatal errors.
      * @hide
      */
+    @SystemApi(client = SystemApi.Client.MODULE_LIBRARIES)
+    @Nullable
     public static IBinder waitForService(@NonNull String name) {
         return Binder.allowBlocking(waitForServiceNative(name));
     }