Add API to request light navigation bar in theme

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

Test: CtsSystemUiTestCases
Bug: 66986286
Bug: 63630024
Change-Id: I0b89c58e9fbca08e27a6ec13db5a07f5e030c682
Merged-In: I81b34fefeb336ac2269dc5e6b98cc0b6045cea70