Support multiple dark tint areas in status bar

- With ag/16678942 the system will now compute multiple appearance
  regions for all windows under the status bar, so we need to handle
  scenarios where multiple apps can create multiple dark tint areas

Bug: 208717544
Test: atest SystemUITests:LightBarControllerTest
Test: Install MultiAppeareanceRegionTest apk from b/208717544 and
      verify that that the multiple dark areas work in multiwindow
Change-Id: I0d8696f6b2fb7793ccbdcbfe7e2520fceb24e5ec