Create an interface for NPVC

This only creates the interface. References to NPVC will be replaced
in subsequent changes in the chain. Every public method was checked
for inclusion in the interface, and some method scopes were reduced
instead. Some methods were renamed, and some were deleted.

Bug: 273789810
Test: Manually checked go/shade-cujs
Test: atest SystemUITests SystemUIGoogleTests
Change-Id: I8d97e430b44aae76b5ab38faaa9b5ce81d2074e8