Use status bar appearance to determine header element brightness.

Currently we use status bar color to determine header element
brightness, which has the side effect of defaulting to bright elements
on a clear caption. This CL will instead use the
WindowInsets.APPEARANCE_LIGHT_STATUS_BAR flag to determine brightness in
this particular case.

Bug: 280833361
Test: Manual; open apps with and without clear status bar.
Change-Id: Ie429825a76c20a5e1767bb80ba99cae8b7ce6068