Try to refresh subs credential on startup

[simonhong]

Resolves brave/brave-browser#26607

If cached credential is expired at startup, current user could be
purchased user but it could be expired while terminated.
So, trying to refresh credential by reloading purchased state at
startup.

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

BraveVPNServiceTest.CheckInitialPurchasedStateTest

Manual test - As it's hard to wait till cached credential is expired, modifying user data would be much easier for testing.

1. Launch Brave and prepare as purchased user
2. Terminate and modify brave.brave_vpn.subscriber_credential.expiration from Local State file at user dir - change it to lower value (ex, delete least digit)
3. Launch Brave again and wailt several minutes for giving sufficient refreshing time
4. Open vpn panel and check vpn panel shows main page

[spylogsster]

++

[stephendonner]

Verified PASSED using

Brave	1.47.56 Chromium: 107.0.5304.91 (Official Build) nightly (x86_64)
Revision	3d5948960d62418160796d5831a4d2d7d6c90fa8-refs/branch-heads/5304@{#1097}
OS	macOS Version 11.7.1 (Build 20G918)

Steps:

1. installed 1.47.56
2. launched Brave
3. opened brave://flags
4. set brave://flags#brave-vpn to Enabled
5. relaunched Brave
6. loaded account.bravesoftware.com
7. entered username/password from 1PW
8. used [email protected], clicked Get login link
9. clicked the link in the email
10. clicked Browse plans
11. scrolled down
12. clicked on Buy now for Brave VPN Subscription
13. completed the Stripe purchase flow
14. confirmed I could connect to the VPN
15. shut down Brave
16. disconnected from the BraveVPN connection
17. opened Local State from my user profile
18. edited the value for brave.brave_vpn.subscriber_credential.expiration, from 13312583928000000 to 133125839280000 by deleting the last 2 0 numbers
19. relaunched Brave
20. confirmed I could connect to VPN (had to click the Retry button)

Local State	Brave VPN connected