useLedgerLive setting isn't always in sync

This is fairly simple to reproduce:

1. Connect a Ledger account to MetaMask
2. (eth-ledger-bridge-keyring injects the github pages iframe with correct `useLedgerLive` setting)
3. Use the test dapp to perform a signature
4. Remove the Ledger account from MetaMask
5. Add the account again

The `useLedgerLive` setting will be false in the IFrame, leading to the user's browser hanging (while the iframe incorrectly tries U2F.

Presently MetaMask sends the `useLedgerLive` setting down to the iframe when the user logs in, and when they toggle the setting. That isn't reliable enough in some cases.

Start by reproducing the five-step flow with a Ledger account and the test dapp, then trace how MetaMask sends useLedgerLive to the GitHub Pages iframe during login, toggling, removal, and re-addition. Done means the iframe retains the correct setting after the account is added again and does not incorrectly attempt U2F or hang the browser.

Written by the indexing model from the issue text.