After tapping "Try another account", uninitialized tab bar UI is shown briefly before login prologue screen

[jaclync]

Describe the bug

Similar to #3107, presenting login UI from WPAuthenticator library tends to have some noticeable delay. In this case, it is when the user decides to restart the authentication flow by tapping "Try another account" on the store picker and the app dismisses the login UI then presents login UI again. The reason why we can't just reuse the login UI and go back to the first screen is because the prologue screen might not exist anymore when logging in with magic link (#3423).

To Reproduce
Steps to reproduce the behavior:

1. Start logged out of the app
2. Select the WordPress.com login option
3. Log in with email and password or magic link
4. On the store picker screen, select "Try another account" to log in to a different account --> the uninitialized tab bar UI is shown for a split second before the prologue screen is shown

Screenshots

simulator.mov

Expected behavior

The uninitialized tab bar should not be shown when presenting the login UI.

Isolating the problem (mark completed items with an [x]):

• I have deactivated other plugins and confirmed this bug occurs when only WooCommerce plugin is active.
• This bug happens with a default WordPress theme active, or Storefront.
• I can reproduce this bug consistently using the steps above.

Mobile Environment
Please include:

• Device: iPhone 11 sim, iPhone 6s Plus
• iOS version: iOS 14.1
• WooCommerce iOS version: 5.7