Importing Brave Payments from Brave (old) is not working

[bsclifton]

Description

Sometime after brave/brave-core#736 was merged to master, the behavior of the ledger API seems to have changed. This causes a situation where the import fails

Steps to Reproduce

1. Fresh profile
2. Build from source (master)
3. Have a Muon profile setup, with a Brave Payments wallet (with a balance)
4. Launch Brave Core
5. Visit chrome://settings/importData, pick Brave (old), and only choose Brave Payments
6. Run import process

Actual result:

Calls appear to happen in this order:

1. Fresh profile; profile write calls CreateWallet
2. GetWalletProperties (handler) is called BEFORE CreateWallet finishes
3. GetWalletProperties shows a balance of 0; it attempts to backup the wallet (basically copying the ledger_state file to something like ledger_import_backup_1543557117056)
4. The CreateWallet call (from step 1) finally finishes- RecoverWallet is now called
5. The backup kicked off by step 3 finally gets executed; it sees a positive balance (because step 4 has executed). This cancels the import

Expected result:

The code should be robust enough to handle observers firing out of order. For example, in the above, step 2 should be skipped because the wallet hasn't been created yet.

Reproduces how often:

100%

Brave version (brave://version info)

[bsclifton]

Adding QA/No as this is already covered with test plan in brave/brave-core#736