Revert "Allow views to setSystemUiVisibility() with the same value and have it work."

This reverts commit fad95552.
Because it just didn't work. (The views kept aggressively
re-applying their preferred flags every time view attributes
were aggregated.)

We'll pursue the original strategy, like lights out mode in
HC: if you want to tickle this mode, un-set and re-set the
flags (preferably synchronized with the global SystemUI
visibility state as reported by onSystemUiVisibilityChanged
callbacks).

Bug: 5052456