Finalize authentication that was started via beginAuthenticationAsync.
If authentication wasn’t started via this manager we won’t accept this authentication attempt,
false. This may happen if WebChannelFeature is enabled, and user is manually
logging into accounts.firefox.com in a regular tab.
Guiding principle behind this is that logging into accounts.firefox.com should not affect logged-in state of the browser itself, even though the two may have an established communication channel via WebChannelFeature.
Return A deferred boolean flag indicating if authentication state was accepted.