Uses a provided fromAccount to sign-in into a corresponding FxA account without any required user input. Once sign-in completes, any registered AccountObserver.onAuthenticated listeners will be notified and authenticatedAccount will refer to the new account. This may fail in case of network errors, or if provided credentials are not valid.
reuseSessionToken - Whether or not to reuse existing session token (which is part of the ShareableAccount.
Return A deferred boolean flag indicating success (if true) of the sign-in operation.