Wallet info can be lost during upgrade in very specific edge case

[bsclifton]

Test plan

1. Have a v1 wallet (created with 0.18.36 or earlier)
2. Upgrade to new version of Brave
3. When launching Brave, you'd need to exit after sync is called but before transition finishes
   • once you see the overlay, quit out. Doesn't have to be immediate, you can probably quit out 5 - 10 seconds after seeing it.
4. View that the ledger-newstate.json file exists in your profile directory (ex: ~/Libraries/Application Support/brave-folder-here/ledger-newstate.json. It should contain key/pair values (an array of numbers)
5. re-launch Brave and go back to about:preferences#payments
6. Verify transition completes
7. Verify that the ledger-newstate.json file is no longer in your profile directory (ex: ~/Libraries/Application Support/brave-folder-here/ledger-newstate.json

Description

The keypair used by the client isn't generated until after the first call to ledgerClient.sync. We persist before sync and don't ever re-persist

Steps to Reproduce

1. Have a v1 wallet (created with 0.18.36 or earlier)
2. Upgrade to new version of Brave
3. When launching Brave, you'd need to exit after sync is called but before transition finishes
   • once you see the overlay, quit out
   • re-launch Brave and go back to about:preferences#payments

Actual result:
Upgrade can potentially cause loss of funds (if upgrade finishes, nothing receives the new wallet details)

Expected result:
Wallet details should be persisted so that a reload of Brave is safe and doesn't trigger another transition

Reproduces how often: [What percentage of the time does it reproduce?]

Brave Version

about:brave info:

Brave: 0.19.48
rev: de939f6
Muon: 4.4.28
libchromiumcontent: 61.0.3163.100
V8: 6.1.534.41
Node.js: 7.9.0
Update Channel: Release
OS Platform: macOS
OS Release: 16.7.0
OS Architecture: x64

Reproducible on current live release:
yes

Additional Information