Mnemonic stays in memory even after user logout if it's restored from backup

[kwunyeung]

Generic information

• OS name and version: iOS 13.6
• Application build code: 4014
• Network type (mobile/Wi-Fi): WiFi

Bug description

The mnemonic phrase was not cleared after restoring the key from backup. It stays in memory. After I logged out and tap "I have a written mnemonic", it will shows the exact same mnemonic of the account I restored from backup.

Steps to reproduce

1. Export mnemonic with password
2. Log out
3. Login with "Use mnemonic backup"
4. Paste the encrypted backup and input the password
5. Log out again after login successful
6. Login with "I have a written mnemonic"

The previous mnemoinc will be shown on the 24 blanks.

Expected behavior

The 24 blanks should be all clear in all situations.