Brave Rewards opt-in ("Start using Brave Rewards") should go through Rewards panel

[Miyayes]

Description

Currently, if a user does not have Brave Rewards enabled and goes into brave://rewards (Rewards settings page), they can press to "Start using Brave Rewards". Then, Rewards is enabled, and onboarding appears as a modal.

Solution

• [ ] Pressing "Start using Brave Rewards" should just pop open the Rewards panel with its opt-in flow
• [ ] Remove all in-page Rewards panel elements from brave://rewards while user is still not opted in, and center the Rewards opt-in component

[Miyayes]

Should be completed before/in conjunction with https://github.com/brave/brave-browser/issues/24542.

[Miyayes]

For Android, make sure it opens the natively implemented Android Rewards panel.

[GeetaSarvadnya]

Verification is inprogress

Brave | 1.45.75 Chromium: 106.0.5249.65 (Official Build) beta (64-bit)
-- | --
Revision | 3269dc3633cdd2ab94546fdbe54962e45b17a6e0-refs/branch-heads/5249@{#580}
OS | Windows 10 Version 21H2 (Build 19044.2006)

• Verified the test plan from https://github.com/brave/brave-core/pull/14858
• Confirmed that the rewards opt-in go through the rewards panel when click on Start Using Rewards button in both rewards settings page and NTP

brave://rewards->Click on Start Using Rewards button

NTP->Click on Start Using Rewards button

• Installed 1.44.x and upgraded the profile to 1.45 (by renaming the profile folder to beta) and opened brave://rewards and clicked on Start using Brave Rewards button and ensured that the Rewards panel is opened with onboarding displayed

[LaurenWags]

Verified with

Brave | 1.45.87 Chromium: 106.0.5249.91 (Official Build) beta (x86_64)
-- | --
Revision | fa96d5f07b1177d1bf5009f647a5b8c629762157-refs/branch-heads/5249@{#707}
OS | macOS Version 12.6 (Build 21G115)

Verified test plan from https://github.com/brave/brave-core/pull/14858#issue-1352704849.

• Confirmed when on brave://rewards (prior to opting into Rewards), when the "Start using Brave Rewards" button is clicked, the panel opens and presents the option to join rewards.
• Confirmed you are not opt-ed in until you select "Start using Brave Rewards" from the panel.
• Confirmed once you select "Start using Brave Rewards" from the panel, Rewards page content is displayed.

Example	Example	Example

Additionally, spot checked other areas to ensure that the panel is opened for enabling Rewards/onboarding:

• NTP Widget
• NTP SI logo popup
• brave://settings/rewards
• Customize Dashboard settings
• Twitter in-line tip button
• GitHub in-line tip button
• Reddit in-line tip button

Example	Example	Example	Example	Example	Example	Example