Simplify Purchased step on desktop

[simonhong]

Test plan

Basically make sure happy path is good to go

1. Fresh profile
2. Click VPN button
3. You should see sell panel
4. Visit account.brave.com and do a purchase
5. Purchase should work and load credentials
6. Click VPN button
7. You should see regions and toggle for VPN on/off

Description

On desktop, we're waiting to enter Purchased status when we have valid region data after getting subscriber credential.
We do this because region data is needed to show vpn main panel.
However, region data is not related with purchasing and it makes purchased status handling complex.
Also, because of that we have many ifdef android because android doesn't need region data from BraveVpnService.

To make it more simpler, we could enter Purchased state after getting subscriber credential.
and region data handling is done by BraveVPNOSConnectionAPI.
As region data is used for connecting, BraveVPNOSConnectionAPI would be good place to manage region data.

VPN panel can show sell view when it's not purchased state yet.
and VPN panel can show main view when BraveVPNOSConnectionAPI have valid region data.

[kjozwiak]

The above requires 1.51.110 or higher for 1.51.x verification 👍

[stephendonner]

@simonhong mind giving us a unique testplan for this one? What's written so far in the PR for brave/brave-core#18009 (comment) seems to apply more to #29728.

And/or if we already verified #29728 (comment), can that verification apply? Thanks!

[bsclifton]

@stephendonner added test plan! 😄

And yes! #29728 (comment) should be perfect

[stephendonner]

Verified PASSED using

Brave	1.51.110 Chromium: 113.0.5672.77 (Official Build) (x86_64)
Revision	c4236862955e005c2187105415ac4a2ecf86dff1-refs/branch-heads/5672_62@{#3}
OS	macOS Version 13.4 (Build 22F5059b)

Followed the testplan in #29520 (comment) and covered it via #29728 (comment)