Add slide to Rewards onboarding flow in Japan for wallet verification

[Miyayes]

Description

Add additional slide to the Rewards onboarding flow in Japan for wallet verification.

• Slide will be the very last in the onboarding, even after the "Woohoo you are all set!" slide.
• Buttons: "Do it later", and "Verify my wallet"
• "Verify my wallet" button should take the user to the OAuth flow, just like clicking "Verify wallet" in Brave Rewards settings

[stephendonner]

Verified PASSED using

Brave	1.24.65 Chromium: 90.0.4430.72 (Official Build) dev （x86_64）
変更履歴	b6172ef8d07ef486489a4b11b66b2eaeed50d132-refs/branch-heads/4430@{#1233}
OS	macOS バージョン11.2.3（ビルド20D91）

Steps:

1. set region and language to Japan/Japanese, respectively, via the OS
2. create a new profile for Brave
3. launch using ./Brave\ Browser\ Dev --enable-logging=stderr --vmodule="*/bat-native-ledger/*"=6,"*/brave_rewards/*"=6,"*/bat-native-ads/*"=6,"*/bat-native-confirmations/*"=6,"*/brave_ads/*"=9,"*/brave_user_model/*"=6 --brave-ads-staging --brave-ads-debug --rewards=staging=true,reconcile-interval=4
4. load brave://flags and set Enable bitFlyer for Brave Rewards to Enabled
5. manually close the browser, and relaunch using the command-line (clicking the Relaunch button via brave://flags will obviate the --rewards=staging=true, which is necessary)
6. click on the BAT icon
7. peruse the Brave Rewards onboarding tour
8. on the last slide, click on the purple button
9. in the resulting purple panel with the orange button, click on the orange button
10. look at the resulting page

Confirmed I was taken to the OAth page for BitFlyer, to log in and confirm my wallet: https://demo24kiuw4dcyncsy3qlud8u8.azurewebsites.net/en-us/login?backurl=%2fex%2fOAuth%2fauthorize%3fclient_id%3d078bf3871f3741432e813376d996a6a0%26scope%3dassets%2520create_deposit_id%2520withdraw_to_deposit_id%26redirect_uri%3drewards%3a%2f%2fbitflyer%2fauthorization%26state%3dA31969F595A977854D789DA66E33A64D52B443DE59F31E9D3CA996CD09E38B32%26response_type%3dcode%26code_challenge_method%3dS256%26code_challenge%3d1pAhqBvPcrOffFaTWy0Fdxk_w18QN1RVcA8qs_A15fE

example	example	example	example

[srirambv]

Verification passed on OnePlus 6T with Android 10 running 1.24.82 x64 build

• Verification notes can be found here

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Verification passed on Samsung Tab A with Android 10 running 1.24.82 x64 build

• Verification notes can be found here