Fix possible crash involving the main ProgressBar.

Slightly refactored and centralized access to the progress bar from
various fragments. Also, posting the progress bar updates to the message
queue, since it could be called from within Activity.onCreate, where on
some devices the progress bar is not yet created.

Bug: 72817

Change-Id: I021123193c8942af52011f1e9c6f347b9c28ed49