Move login open event tracking from showLogin to view controller's viewWillAppear

[jaclync]

Idea from @diegoreymendez: wordpress-mobile/WordPress-iOS#15687 (comment)

Currently, .openedLogin event is tracked in showLogin when the source view controller is presenting login UI:

WordPressAuthenticator-iOS/WordPressAuthenticator/Authenticator/WordPressAuthenticator.swift

Line 163 in cc79aca

trackOpenedLogin()

Now that we have a separate function loginUI for the host app to display login UI, the host app is responsible for tracking .openedLogin event but it could be easily missed. Let's move this event tracking to login view controller's viewWillAppear so that the host app doesn't have to manually track the event using both showLogin and loginUI.