Always remove unhide flags while pretending callback from SystemUI

Logic to show system bars in BarController.adjustSystemUiVisibilityLw
requires that the current system UI visibility doesn't contain the
unhide flag. The unhide flags were cleared by the SystemUI, but it won't
callback any system UI flags anymore. So this CL also removes the unhide
flags while pretending callback from SystemUI.

Bug: 143135605
Bug: 118118435
Test: Manually test with fullscreen apps
Change-Id: I9552ff9ff8ca778667cd02a354b2febf912e899e