Progress thread: less aggressive redraw

In commit I643e30505f962ebbf67aae57baa06a296006e542 , 'enhance the
progress thread update', I incorrectly amended philz-cwm6's commit to
work around a theoretical issue of keeping the installer's spinning
cube rotating when no progress bar is shown. While my amendment works,
it keeps the progress_thread updating text and overlays even when
sitting at the menu. This is overly aggressive. Change the update
mechanism to only apply when the background installer image is visible.

Also, make ui_increment_frame() static since is is local-only now.

Change-Id: I2fa2f8a0704f562fb027ba4bee9b1b7d278da404