Condition display of progressbar on a isCreatingAccount LiveData

Avoiding the mess with saving onSaveInstanceState, and the (in this case) unwanted restoring of it upon back-button tap.

Closes #1919 (closed)

Test instructions:

• Precondition: fresh install, setting up a new account
  • Testing specific bug fix:
    • Choose a name, tap next
    • Choose a password, tap next
      • Not testable on some devices which display "Create account" instead of "Next"
    • You are now on Background connections screen
    • Tap Back-button ◁
    • Ensure that password can be changed again
  • During setup process, rotate device and ensure that:
    • entered text is kept
    • progressbar is continuously displayed

Please also verify that the following is fixed and can be closed: #1819 (closed) (Create Account-button should never visible when account is already being created)