Remove temp GAIA education screen from PS setup code

With this change the temp GAIA education screen is removed from private
space setup code and adds a new loading screen with "Just a sec" title
in the activity while the gms intent in launched to fix blank screen issue.

Screenrecording link:
b/336822960#comment7

Bug: 336822960
Test: Manual
Change-Id: I9ee41835b39c194fefb970321b7a9d5a30f87306