Adapt to removal of base::DictionaryValue::SetDictionary() method II

Use base::Value::SetPath() instead.

Chromium change:

https://source.chromium.org/chromium/chromium/src/+/61bc6c4bf56e93687fed13327c02e0addc97a0c4

commit 61bc6c4bf56e93687fed13327c02e0addc97a0c4
Author: Alex Cooper <alcooper@chromium.org>
Date:   Tue Jan 4 22:53:42 2022 +0000

    [CodeHealth] Remove deprecated SetDictionary method

    The SetDictionary method is deprecated. Now that all callers of it have
    been removed, it can be removed as well.

    Bug: 1187029

browser/brave_rewards/android/brave_rewards_native_worker.cc

@@ -261,9 +261,9 @@ BraveRewardsNativeWorker::GetWalletBalance(JNIEnv* env) {
   base::DictionaryValue json_root;
   json_root.SetDoubleKey("total", balance_.total);
 
-  auto json_wallets = std::make_unique<base::DictionaryValue>();
+  base::Value json_wallets = base::Value(base::Value::Type::DICTIONARY);
   for (const auto & item : balance_.wallets) {
-    json_wallets->SetDoubleKey(item.first, item.second);
+    json_wallets.SetDoubleKey(item.first, item.second);
   }
   json_root.SetPath("wallets", std::move(json_wallets));
   base::JSONWriter::Write(json_root, &json_balance);