Add @Deprecated annotations to Firebase App Check SafetyNet SDK public API.

Author: rosalyntan

appcheck/firebase-appcheck-safetynet/src/main/java/com/google/firebase/appcheck/safetynet/SafetyNetAppCheckProviderFactory.java

@@ -24,6 +24,7 @@
  * Implementation of an {@link AppCheckProviderFactory} that builds {@link
  * SafetyNetAppCheckProvider}s. This is the default implementation.
  */
+@Deprecated
 public class SafetyNetAppCheckProviderFactory implements AppCheckProviderFactory {
 
   private static final SafetyNetAppCheckProviderFactory instance =
@@ -35,6 +36,7 @@ private SafetyNetAppCheckProviderFactory() {}
    * Gets an instance of this class for installation into a {@link
    * com.google.firebase.appcheck.FirebaseAppCheck} instance.
    */
+  @Deprecated
   @NonNull
   public static SafetyNetAppCheckProviderFactory getInstance() {
     return instance;