chore: Update to use local Builder class

Author: lqiu96

oauth2_http/java/com/google/auth/oauth2/AwsCredentials.java

@@ -234,8 +234,8 @@ public static AwsCredentials.Builder newBuilder(AwsCredentials awsCredentials) {
   }
 
   @Override
-  public AwsCredentials.Builder toBuilder() {
-    return new AwsCredentials.Builder(this);
+  public Builder toBuilder() {
+    return new Builder(this);
   }
 
   public static class Builder extends ExternalAccountCredentials.Builder {

oauth2_http/java/com/google/auth/oauth2/ExternalAccountCredentials.java

@@ -683,7 +683,7 @@ private static boolean isValidUrl(String url) {
   }
 
   @Override
-  public abstract ExternalAccountCredentials.Builder toBuilder();
+  public abstract Builder toBuilder();
 
   /**
    * Encapsulates the service account impersonation options portion of the configuration for

oauth2_http/java/com/google/auth/oauth2/IdentityPoolCredentials.java

@@ -154,8 +154,8 @@ public static Builder newBuilder(IdentityPoolCredentials identityPoolCredentials
   }
 
   @Override
-  public IdentityPoolCredentials.Builder toBuilder() {
-    return new IdentityPoolCredentials.Builder(this);
+  public Builder toBuilder() {
+    return new Builder(this);
   }
 
   private IdentityPoolSubjectTokenSupplier createCertificateSubjectTokenSupplier(

oauth2_http/java/com/google/auth/oauth2/PluggableAuthCredentials.java

@@ -204,8 +204,8 @@ public static Builder newBuilder(PluggableAuthCredentials pluggableAuthCredentia
   }
 
   @Override
-  public PluggableAuthCredentials.Builder toBuilder() {
-    return new PluggableAuthCredentials.Builder(this);
+  public Builder toBuilder() {
+    return new Builder(this);
   }
 
   @VisibleForTesting