Add API to request light navigation bar in theme

Such that splash screens can already have a light navigation bar,
avoiding a janky flash.

This is the CL on master for this change, which only cleans up the
API.

Test: CtsSystemUiTestCases
Bug: 66986286
Bug: 63630024
Change-Id: I81b34fefeb336ac2269dc5e6b98cc0b6045cea70