Fix invisible status bar icons when no appearance region reported

After CL[1], it checks all overlapping windows with the status bar
before reporting appearance regions to SysUI. If there's no overlapping
window, it'll report empty appearance region. This prevents setting the
foreground color of the status bar to dark unexpectedly by checking if
there's any light appearance regions reported first.

[1]: Id69a8dadcf9044fb2cde1b50108479a4375332be

Bug: 198972446
Test: launch immersive apps with layoutInDisplayCutoutMode=never,
      observed the foregound color of status bar is light while the
      background color is dark.

Change-Id: I7c1d2fb72f38f0a67d982d5e8509cf5a9f3b98d2