Ensure no duplicates in ThemeKey

Some apps keep adding the same resources to their theme key
objects on e.g. switching dark mode. Need to be prepared to
limit the size of the arrays, otherwise theme switching gets
slower with each toggle

+ a bit more efficient native handling

Bug: 242005877
Test: manual, 10k of theme switches with no noticable slowdown
Change-Id: Icf74770bd41ebeb0a31f527ae3616de00f23b0ae